I'm thinking that maybe the aggressive cat is getting the least
attention and maybe feels like you favor her least.
To help your aggressive cat to get along with his buddy start to
talk with him more especially when the other cat is around to observe
your acceptance of the other cat which is ussualy the is usually
upsetting you because you don't know how to stop him from being
aggressive .
Also you can play with one of them when the other cat is present and
encourage them to be playful.