Friends turned enemies

Hi all,
To make a long story short, I was fostering 2 strays in my kitchen this winter when one of the cats snuck into our living room (thanks to my DH not shutting the door properly) and attacked my resident cats. Since then, my 2 resident cats have been fighting.
It has been 2 months and they have still not made up. I've got them in separate bedrooms and have to let one out at time. I have tried feliway, rescue remedy, switching blankets, cages, leashes, screen doors, you name it. It seems to be my female Xanadu that has the problem. Gizmo (male), for the most part, is afraid of her and creeps around if he thinks she's near. I have gotten them to the point where they can eat together, but thats it. Xanadu will walk up to Gizmo's door and hiss at it. When I try to keep them together (after eating) Xanadu will start hissing and growling and swatting at Gizmo. Gizmo will ignore it at first and even walk away, but Xanadu gets louder and more aggressive and then Gizmo will crouch down, put his ears back and attack!!
I am devastated. How long should I keep this up? Is there relationship damaged beyond repair? Is there anything else I can do?

:( So sorry to read of your problem.. Now are the forster babies still in the kitchen?? What sex are these forster babies?? Could be your female resident cat smells the forster cats & she is taking out her disapproval of them towards your male resident cat.. You may have to give up the idea of forstering.. Also what age are the different kitties??

The foster kitties have been moved to my mudroom. My kitchen separates the mudroom from the rest of the house. My cats are not allowed in the kitchen (remodeling) so my resident cats cannot interact, see or hear the fosters. The fosters have no other place to go, so I have to keep them somewhere. The fosters are 1 adult female (aged about 5) and 1 female kitten aged 7 months).
My cats are 1 female Xanadu aged 6 and 1 male Gizmo aged 6. They have grown up together.
All are spayed/neutered.