If you're talking to your vet anyway, talk about the fighting. Maybe some kennel drops for a while will help the one cat.

Maybe a behaviourist can help - if it started as soon as the new kitty arrived, maybe not.

I can understand how hard it is to keep them separated in an apartment...so maybe they didn't have enough separate time right at the beginning.

I know someone on PT will have good advice!

hugs
Catty1