The New FFIV

I want to have certain characters have certain abilities like Twin Cast, but I can't get them to work really so I'm putting it on hold.

This is a interesting idea. To make this thing work properly, you could create a new reversible and hidden status, with a timer (like some status such has comdemned). Your two characters who have the status make the animation pose. The twin cast could be in fact a single cast and you reset the turn of the other player if everything succeed after the timer reaches 0. If any of the player lose the status (by mute or death), the spell is canceled like any other spell when the character is muted between the selection and the cast.

You could make some spell stronger if the caster has the hidden twin status or make single targetting spell become multi-targetting for a single cast. There could be a way also to make some spell twin exclusives and grey out the spell(s) in the magic or lore menu.

All this is a lot of ASM in perspective but if there is FFIV dissasemblies that exist or documentation about the twin command maybe you could see how it's made in FFIV.