fighting sisters

Question
I have two spayed sister cats. I rescued them about 2 or so years ago and they were the best of friends. A few months ago my one cat was in the window sleeping and was spooked by a large(maybe male) cat outside. I scared the cat away and about 20 minutes later it came back when both of my girls were in the window. They ended up attacking each other to the extreme! They are still fighting even months after this whole ordeal. I have tried everything to make them live in harmony but its not working.Seperating them, spraying them with water, putting the one in a cage with the other one free and vice versa, you name it, I probably tried it. So my question is this, would seperate litter boxes make a difference? I havent tried this one yet. Any helpful advice would be appriciated!!! Thanks!

p.s Giving one away isnt a option.

Answer
Hi Katie,

First off make sure they can't see out of that window anymore. This is what is called redirected aggression. Since they can't get at the strange cat they go after each other. You might want to try some Feliway comfort zone. This mimics a cat's happy pheromones and can calm them down. Try a cat introduction protocol. This will help. Introduce them gradually to each other again. Praise and stoke each one when they are together. Feed them special treats. You want to make the time when they are together as pleasurable as you can. Separate litter boxes might help. This gives each cat their own space.  

It is imperative that they don't go back in that window. You don't want this whole thing to start again. If worse comes to worse, take them to the vet for some tranquilizers. This will help. I would also make sure that when I go to the vet with one I would take the other. Redirected aggression can stem from that too. Good luck.

Ciao, Karen