I've been having a very hard time integrating 3 cats in my home. They're all rescues and have no other option than back to the streets if this doesn't work out.
I've had very little advice that worked, as these situations are very complex and each case so unique.
The things I've learned (the tough way) are to learn from the cats themselves what will work and what won't. If we pay very close attention, we see they do show us. Also: try to be calm yourself, but really from within, find the strength to just let it be, tomorrow may be a better day, and good days may become more the norm. Last, make sure they have vertical places to escape to, apart from the horizontal usual ones such as another room.
In your situation, I might just let them be. Leave all doors open, let them find their safe places, not "put them" here or there. Prince and Princess have found a status quo where the bedroom is only hers, unless the three of us are in it. She doesn't go out of the bedroom, because he attacks her (still, a princely life between soft flannel, feather duvets, the best toys, lavish love and attention, sleeping curled up with mommy all night, and gourmet food, compared to weathering the storms in the streets, hungry, cold, wet, with a skin infection, lonely and MORE scared).
Last week I got a third cat, and things are far from easy. Right now it's doing all I can to avoid having to put her back in the street. I'm watching VERY closely for any signs of things that make things better, to do more of that, and things that make things worse, to prevent those.
One other thing I've learned is that sometimes it's the attacking cat that is more in need of reassurance, not the victim, that's why he's attacking.