Headcanon:
Since Zib can't be the true "Number 1 Zim" because he isn't a Zim at all but a Dib, that makes Number 2 the actual Number 1 Zim. Following this idea, I theorize that where Zib is the only Dib to have defeated his Zim, Number 2 is the only Zim to have defeated his Dib and conquered earth.
This gives them a unique connection because they're the only versions of each other who can relate to one another's complicated feelings of regret and longing for the past. Zib couldn't handle destroying the only person who understood him and gave him a purpose, only for no one to care, so he merged with Zim's PAK as the only way to be with him again. Number 2 went through the same thing, destroying his Dib for the Empire only to learn the Tallest never cared about his mission. But since there was no way for him to get his Dib back, he coped by joining Zib willingly to get revenge on the Irken Empire, and was the only one to know the truth about "Number 1".
If you're wondering how Number 2 can be unique as the only successful Zim when there's another Zim who won in an earlier comic, well...
At the end of that comic, Zim claimed that defeating himself in the future somehow undid the past, although Dib pointed out that made no sense. But in the Zimvoid, following another train of non-logic, Zim erased every alternate version of himself by feeding them packing peanuts to merge their timelines, which both Dib and Zib point out makes no sense. But since the latter ended up working out due to Rule of Funny, I'm inclined to think the former did as well. Except it actually does hold up to logical scrutiny when you think about it, just not for the reasons Zim thinks.
Earlier in the Future Zim comic, Zim tried to find out from his future self which of his evil plans was successful, but the only thing he finds out is that it wasn't the one where he covered the earth's atmosphere in cheeto dust. Later, at the start of the Virooz arc, we see Zim attempting this very plan despite being told it wouldn't work. So either Zim was so enamored with the idea he blocked out all memory of being told it would fail, as he usually does whenever someone tells him something he doesn't want to hear, or he deliberately attempted a plan he was told wouldn't work just to spite his future self. Either way, meeting his future self altered the course of Zim's destiny because meeting him caused him to make different choices, thereby rewriting the future and erasing Future Zim's timeline. So Future Zim no longer exists by the time of the Zimvoid adventure. Then the timeline branched at some point, creating a new Zim variant who instead of conquering the earth and then putting on a fake beard and high heels somehow grew taller naturally and could throw Vibrating Irken Death Blades with 99.4% accuracy.
I've theorized before that because timelines are always splitting Zib can never truly be erased. After the Zimvoid adventure, ETF came out and created a new timeline that split off from the first comic, creating a universe where the events of the Zimvoid haven't happened yet and therefore Zim hasn't gone back in time to feed everyone packing peanuts to erase them, thereby restoring Zib's existence. I think the nature of the multiverse pretty much guarantees that there will always be a version of Dib who wins and a version of Zim who wins. Zib always comes into existence and creates the Zimvoid while Victorious Zims take many different forms, Future Zim and Number 2 just being two of the infinitely possible examples.
