I only have 2 girls not seperated and they dont fight At All. i got them both younger and smaller and threw em in together and they do just fine. My smaller one follows the bigger one (not by much on size) and they do lots together. Sometimes(rarely) the bigger one will push the other one away but no nipping just like a little headbutt and the other one will just swim away not even scared.
It would be safer just to divided the tank. Two female bettas can be like two male bettas. They might fight to the death D: Just because they don't fight in the beginning, doesn't mean that one day they won't fight!