Forgive my simplifying this or not understanding the nuances as well as I could have, but with what seems to be so much historic, linguistic, and cultural overlap (at least with certain touchstones), is there a reason Catalans and Valencians have branched in identity?
Hello! Sorry for the delay in answering. I've been very busy so I don't have time to add sources for everything as I always try to do, but I'll give you the terms so you can do your own research.
The identity of being Valencian, Catalan, Balearic, etc or from more specific areas like Penedès, Pyrenean, etc don't necessarily mean these people don't also identify in general terms as Catalan (Catalan Countries). Most people identify with different levels. For example, this is how I would say my identity: I am from my town, I am from Penedès (the cultural area within Catalonia), I am from Catalonia, I am Catalan from the Catalan Countries, and I am Mediterranean. They aren't exclusive, all the contrary.
That being said, it's true that there are people (increasing in number) who are Valencian and their Valencian identity has diverged from the broader Catalan Countries identity, or are from Catalonia and their identity as Catalan is limited to Catalonia. In the Valencian case, this tends to be people who don't speak Valencian/Catalan (you'll see why in the post), but there are even some who do.
Catalans and Valencians have historically (since the conquest in 1200s) been part of the same people, as they themselves often talked about back then (they called themselves the same nation). Catalans moved south and settled in what became the Catalan-speaking part of the Valencian Country, and Aragonese people moved south and settled in the inland that eventually became the Spanish-speaking part (eventually Aragonese got substituted by Spanish). The kings were the same in both, families were related in both, the language was/is the same, the books that circulated were in both, etc. It was not perceived as different, but because of how the Catalan political system worked, each area that they invaded was made into its own kingdom: Kingdom of Mallorca and the Kingdom of València were created in this moment. These kingdoms were part of the same system, more or less in a federation with Catalonia and Aragon. (Aragon was always seen as more different because it had its own separate history/legacy and legal privileges.)
Now, fast forward some centuries. After all the kingdoms of this Catalan-Aragonese federation are conquered by the Castillian armies in the War of the Spanish Succession (ending in 1715), all these federated kingdoms lose their independence and become simply part of Spain. When Spain organizes its administrations, it does so under a system of provinces, and the provinces are not allowed to group themselves together (that's why the Mancomunitat de Catalunya in 1914-1923, the first time that provinces were allowed to put together their money and do a few things together, was such an important thing at the time, even though it was almost purely symbolic).
A strong repression follows, which will continue very strongly with Franco's fascist dictatorship (1939-1978). During all of this long time of repression, the Catalanophobic Spanish nationalists tried the "divide and win" strategy, creating theories that said the Catalan Countries have nothing to do with each other and putting us against each other (saying x other area wants to steal your history and claim it as theirs —when in reality it's a common history— and this kind of statements). Some of these movements manufactured by the fascists only convinced a handful of people and never got far, for example the "Leridanismo" movement, which said that the Lleida area (Western Catalonia) isn't actually Catalonia. This will sound absolutely ridiculous to anyone now (and already sounded absolutely ridiculous to most people back then), but when the government is the one deciding education and funding institutions... Well, some times it works. In Lleida it didn't because it has literally zero things to base itself on, but in the Valencian Country it sank in a little bit (not much at first) because there's the (real) claim that the Kingdom of Valencia used to be its own entity (in federation with Catalonia, the Balearic Islands, Aragon, and at times parts of what now is Italy).
Meanwhile, the education system was very reduced and extremely controlled by the government (later, a fascist dictatorship). Many people didn't go to school or went only for a few years, so many people didn't know much outside their town and the city they go to market in. Many people didn't know any history, they knew the language they speak but they didn't know what is spoken where or how far away other places are. This is normal in the kind of society 1940s-1950s Iberia was. So these people would go to school and be taught (by the fascist dictatorship's schools) that Spain is an eternal empire chosen by God to wage the holy crusade etc etc etc and the province division. Thus, the government wanted to make people identify only with Spain, and to a little extent the province they're in, but erase the idea of larger nations that could be stronger together.
Towards the end of the dictatorship and when the dictatorship ended (1978), the Valencian Country was where Catalan independentism was the strongest. For context, the dictatorship ended when the dictator Franco died peacefully of old age in bed (1975) and then starts a period where the fascist government tries to reorganize to keep as many things the same while giving in to a minimum of democracy to be internationally respected as a modern country. In this context, one of their obsessions was to keep the subjugation of the nations (that's why, for example, when political parties were legalized, the only parties that were kept illegal were the ones who were pro-independence). Here enters the strategy to divide the Catalan Countries: blaverism.
Blaverism is a political movement that was funded by the far right-wing Spanish nationalists that claims that the Valencian Country has nothing to do with the rest of the Catalan Countries. It denies the existence of the Catalan Countries, and says that the Valencian language is it's own separate language instead of just the historical term for the Catalan language in the Valencian Country. They deny the term "Valencian Country" because they consider "country" a word that is too strong because, to them, the Valencian Country is nothing more than a Spanish region. They also make a lot of pseudohistorical claims that make absolutely no sense, for example saying that "Valencian" was already spoken there before the Catalan conquest (?????) and some even say that "Valencian" cannot be related to Catalan because while Catalan is obviously a Romance language (=language that comes from Latin) "Valencian" is a language that doesn't come from Latin but from Ancient Iberian itself (????????????? For context, Ancient Iberian is a non-Indoeuropean language that hasn't been desciphered). At this point you're probably wondering if these people can really be that stupid to believe this, and to be fair I don't believe that many people who defend these theories actually believe in them, at least not the "higher-ups". But that's why they need to keep people ignorant. First of all, they took advantage of already-existing Catalanophobia and accentuated it a lot, leading to a new wave of neonazi anti-Catalan(Valencians speakers included) violence during what was called the Battle of València (1970s-1990s). Since everyone who grows up within the Spanish system and society is taught Catalanophobia, many people (including Catalans) can be very Catalanophobic. This internalised Catalanophobia was used to make Valencians more easily convinced of whatever stupid theories they were told, because some of them wanted to believe anything that would tell them they aren't Catalans and they can distance themselves from those horrible, stupid, annoying, arrogant, stingy Catalans.
But now you'll be thinking: "okay, people are willing to believe what will prove them right and make them feel good without having to rethink who they consider worthy of existing or being equal and who they don't. But how can you claim Catalan and Valencian are two different languages, when they're less different than American English is to England English? And when people from Northern Valencian Country speak in the same way as people from Southern Catalonia, while people their dialect is more different from the dialects from other parts of Valencia and other parts of Catalonia?". This is why they desperately need ignorance and still to this day use the law to create it.
On the left: map of the dialects and accents of Catalan. On the right: map of the Catalan Countries with the territorial divisions. You can see how the territorial divisions don't match at all with the dialectal areas, with the exceptions of Northern Catalonia and the islands because of their isolation.
When the dictatorship ended, Spain's system has continued to be based on the region system (called comunidades autónomas). Each region has its own regional government with limited power over certain things (nothing even close to the degree of autonomy that the states of the USA have, or Scotland and Wales in the UK, but it's something). This means that each region has its own of certain things, such as TV channels.
When the TV and radio channels for each region started, you could watch them from anywhere you wanted as long as there were towers/posts that sent the signal there. Since in the Valencian Country, Catalonia and the Balearic Islands we speak the same language, it was common for people to watch the TV channel or listen to the radio they liked from either. This was nice because it meant we had more choice than only 1 TV channel in our language. But this also meant that everyone knew that we speak the same language and can understand each other perfectly. So in 2011, when the PP (right-wing Spanish nationalist) party was in the government of the Valencian Country, they decided to ban Catalonia's TV channel in the Valencian Country, and ban the Valencian TV channel in Catalonia. There were many demonstrations in the Valencian Country and many Valencians tried to keep up pirate signal towers, but the government sent people to saw them down and legally persecuted those who put up the towers. This continues nowadays. With modern TVs, we don't need repetition towers because it works through the internet: we can see the regional TVs and radios of all the regions (Castilla, Madrid, Galicia, Aragón, Extremadura, Cantabria, Andalucía, Murcia, etc)... except for Catalonia's if you're in València, and València's if you're in Catalonia. (I went more in depth about the illegalization and persecution of Catalonia's TV and radio stations in the Valencian Country in this post linked here, it's in Catalan but you can copy-paste it on Google translate if you're interested). This means that now we aren't in communication, we don't follow what happens there are much, and people might become less familiar with other accents. In conclusion, we are being isolated from each other. This way, people are more likely to believe blaverists when they say "those aren't like us", because they don't know what "those" are like.
This "each region have its thing which mustn't be shared with any other" thing also applies to symbols. Each region has its flag. Since the Middle Ages, all the Catalan Countries and Aragon had the same flag (the four red bars on a yellow background, which is the shield of the House of Barcelona). But the Spanish regional system demanded every area to have its own "regional flag". Catalonia kept its historical flag, but the Valencian Country and the Balearic Islands made up a new flag when the region system was being reorganized at the transition from dictatorship to democracy (the Valencian Country's current flag was created in 1982). This new Valencian flag was made by adding a new design of the blue ribbon with a crown on the historical (Catalan) flag. "Blue" in Catalan/Valencian is "blau", thus the term "blaverista" (literally it would mean "blue-erist") because they were the ones who used this new non-historical flag just to be separate from Catalonia. For a few decades, this flag was seen as a very political symbol and you could guess the ideology of those who used it, but over time, after so many years of the governments having it hanging and the newest generations seeing it around as the only flag of their area and studying it in school as the flag of their region, to the young generations it's becoming a less political flag and it's being accepted as simply what the Valencian flag is. This is another very visible example of how shared cultural symbols are artificially separated to make us look more different from each other than we are.
The same isolation applies to other things, including the school system. Each region in the state of Spain studies Spain things in general (history, geography, etc) and some things specific to their region (for example, in Catalonia you learn all the counties of Catalonia and their capitals, in Valencia the same for the counties in Valencia, etc) which means that we are artificially kept apart from the places of our nation that are outside of our region in the Spanish region system.
Now, for all these reasons, it's increasingly more common for people from the Valencian Country to be ignorant about Catalonia (and believe only the stereotypes), and for people from Catalonia to be ignorant about the Valencian Country (and believe the stereotypes). Add to this the huge amount of hatred against Catalonia that the Spanish nationalist political parties rely on in the Valencian Country for votes. As a result, Catalonia is villainized in the Valencian Country, and the Valencian Country is invisibilized in Catalonia.
To sum up, Spanish nationalism keeps us isolated from each other and tries to make us hate each other. This is possible thanks to ignorance and already-existing Catalanophobia, as well as a reinforcement of regional identities and the Spanish identity to counter the Catalan-Countries identity. It's worth adding that this is such a common tactic done by imperialist countries to dilute and kill the local languages that the European Charter for Regional and Minority Languages warns against it and prohibits, among other things, banning peoples from receiving the TV and radio signals of other areas that speak their same language. Spain has ratified this charter but still violates it.
I talked about this topic previously in this answer about the language issue, it might be useful to you:
What do you think about the idea that Valencian is a separate language from Catalan? I'm pretty sure you don't agree with it but do you thin
There could be many more things to say and examples to give, but I think this is enough for now. I hope it makes it more understandable now. Thank you for your interest!
As always, of course, everyone but especially Valencians are more than welcome to comment everything you want to say.












