so i start off with making my two gifs seperatly and colouring them as i normally would, once i've done that i load both gifs into the same project and once i've done all that i started on the grid part:

now go to view -> add guides and this window should pop up:

(it's was so daunting at first when i saw this i was like ????? and started putting in random numbers and was like oh that's how this works 🤣)

here's the settings i used for my gif which is 540px x 500px in size with 5 squares x 4 squares: (if you want more or less you just have to play around with it)

once you've done that it should look something like this:

now the next part is really up to preference again: add a vector mask onto the gif that's above the other one like this: (ignore the name of my folder that is irrelevant 🤣)

and then i used the rectangle select tool (this is because the rectangle select should lock onto the grid squares making it easier to erase certain sections) + a black brush tool to erase the squares to show the other gif that's underneath (you can reposition both gifs to your liking which is what i did)

before & after:

so my vector mask looks like this afterwards

(you don't have to do the squares so close together like i did it was just how i liked it & because of the scenes i had chosen that mine turned out this way)

now onto the grid lines: i used the line tool with these settings

just like with the rectangle select tool the line tool should lock onto the grid line you want to redraw, do this for every line that you want/have for your grid & once you're done go to view -> clear guides and it should look something like this: (added a version with just a black background so it can be seen a little easier)

(i'm ngl idk why my lines ended up kinda faded and not white 🤣 i think it might be because i used a white fill instead of white stroke but it doesn't really matter to me because i got my ideal outcome anyway 🤣)

bonus step: you can stop here if you'd like but i wanted my lines to match my colouring & my intended typography so i put all my line layers into a folder and set the mode to difference & added a yellow fill layer with a clipping mask, like this:

and it should look like this:

you can also play around with the opacity of the lines too which is what i tried out but i prefered for my set the lines being at 100% opacity but it's really up to you with what you want to play around with

now once you're happy with everything merge those layers together (make sure they have the same amount of frames first before merging them) and either save as it is or add some typography like i did and you should end up with this:

for tips when it comes to colouring it really depends on what you're colouring, if you want to manipulate the colours as much as i do i recommend choosing a colour within the scene so you're not having to change too much, or finding scenes that have colours that can be manipulated more easy (any colour that aren't skin tones, unless you're working with red or yellow like i did here, i chose yellow because one of the outfits in the gif was yellow toned and it looked better with the gifset being yellow than my original colour which was blue), also looking for scenes were the people in it don't move as much also is a big help!