I just went through that with my 1 year old rescue female cat and my 3 year old male cat who was there first. I thought they'd never resolve their issues! But they mostly have. I couldn't punish my bully male cat because I work during the day and no one is home with them. I was closing off 2 of the cats in the back of the house and 2 in the front while I was at work but then decided after 2 weeks that it wasn't accomplishing anything so I leave all the doors open now when I'm gone and they're doing a lot better. They seem to have worked it out pretty much. Still a problem now and then but not nearly as bad as it was. Now the female rescue cat seems to be annoying the 4 year old tortie who hisses and spits when she's around. But then she seems to do that now and then anyway to whoever is near her just because she can!

If you can stand the tension, give it a while. Sometimes it takes a few months for them to work it out. It took mine 2-1/2 months to get this far. I just couldn't get into punishing them for their behavior although I know some people can do that successfully. I wish you lots of luck!