I had a similar problem, and eventually I ended up having to seperate the females by rehoming one. One went to my mother (the cat liked her more than me anyways), and the other still resides with me.
When I say they hated eat other I mean, the HATED each other. It started when Mooque was a kitten, and it never got better, it only got worse (yes they were properly introduced). I tried seperating them within the house, feeding them in different places. Nothing worked. Schmutz would beat poor little Mooque, while Schmutz cuddled, mothered and over all babied her brother. Schmutz loves Mister, but hates his sister.
Now that they live apart, and only see each other twice a year (I bring them with me for holidays at my parents house, I can't bear to leave them for more than 3 days without supervision!), they still fight. It's not as bad, I don't hear the blood curdling cries, nor often is their blood drawn. It also takes about 3 days for the real fights to get going, otherwise it's just that they avoid each other. Now though, Mooque is an adult, and is more the agressor.
Schmutz is actually happier now than she's ever been. She is so happy and satisfied being the only cat. Some cats just are better being only cats. We think she thinks she's actually a dog, cos she hangs out with the dogs and acts like them!
My little female can't be near another female, at the vet's they have to cage her with her brother and they can only be next to another male. Some females just hate other females. We've never successfully had a household with more than one at a time. We had 5 neutered boys, one tom and one spayed female, with no problems. But 2 girls? they always try to kill each other!
I do know of people with multiple female cat households with no problems. I think it's an individual cat thing.
I wish I had a better suggestion than this: if it doesn't get better, consider rehoming the less social of the cats. If you have a friend or family member that is familiar with the cat, even better. I am rarely an advocate of rehoming, but sometimes it's better for everyone mentally.