Hello and welcome!
My name is Chris and I am owned by Keiko, a husky/shep mix, and Gunther, my great dane.
One reason I didn't bring any more females into my home is b/c I wanted my current female to remain the dominant (since she is fairly submissive, I knew if I brought in another female she would get bumped down) So I've never had two females before. But, if this fighting happens about every 6 months or so (I'm assuming both are spayed) I don't think it's going to stop. It sounds like they are both fairly dominant females, and both want to be the top dog.
One thing that may help (or may not) would be for you to choose for them who is dominant by feeding that one first, petting that one first and doing everything with that one first before the other. Sometimes dogs just need the alpha (you) to show them where they stand in the pack, instead of them figuring it out for themselves.