I have 2 cats a big fat one (Beni) and a small skinny one (Sundae). I got them a cat tree knowing that they need a space to get away from each other but Sundae is the only one that uses it and doesn’t let Beni get on it. They’ve started to get more aggressive towards each other and idk what to do… I play with them I give them treats not sure what else to do to get them to stop.

Any advise?