Fighting sister!

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
I wouldn't even suggest you giving one away! What is going on here is not uncommon from stray cats being in the yard.

For some strange reason, when cats inside see strays outside they get very territorial and of course the only one they can take their frustrations out on is each other.

There are a couple of options you can try. The first one is Feliway. Feliway is a feline pheromone that makes cats feel like everything around them is peachy and wonderful. It comes in many forms, including something you plug into a socket to spray it in the room all day long.
People have mixed results with it.

The other one is drug therapy for one or both of them. Most vets use Amitriptyline, which is a safe drug that helps with mood. You could try it on one cat or both, but you have to get it from the vet.

Call them and have a consultation with your vet and ask which is the best way to go.
I hope you have some peace soon. I think that as time passes these two will stop fighting with each other but as long as they can see strays outside you might have this problem.

Please let me know how it goes with them in a while.