Why are my cats fighting?

I had the same thing happen with two of my cats I use to have, they were brothers and did literally everything together, even go to the litter box together (a bit creepy if you ask me). They were both really happy until they started maturing. Then they did nothing together, and constantly fought, growled, and hissed at each other. I think it has to do with dominance, normally this is a bigger issue in dogs, but it does commonly happen with cats as well. Your two females might also be in heat and there might be male cats around, and they could be competing for the males, just as males do for females. From my understanding, there isn't much that can be done to stop them from doing this unless you look into getting them both spayed. If that doesn't work you might want to consult a professional to look into the matter. Good luck! :)