K.N.
I am the oldest of 3 girls, and yes we fought alot. but there were times that we got along too. I think that you are doing great by putting them together in the same room, because that way they have to leanr to play (and live together). most kids don't do that anymore and I think that is the biggest problem. if everyone has their own things then they don't have to share or work together. maybe if you get some toys that they can only play with together that might help, especially if you can take them out seperately and discuss what kinds of toys they would like then you can get one that they both like, but that if they want to keep it they have to share it. anyway,I hope this helps.
and yes girls pms their whole lives.