Help, my gerbils keep fighting.?

I have had two female gerbils for around 2 1/2 years, they have never been aggressive to one another before just play fought. Today I woke up to find that both of them had bloodied necks, I bathed them and they seemed fine, but then I saw them fighting again, one is clearly the more dominant and wont stop hurting the other. I've separated them should I keep them that way? or let them fight it out...but wont they kill one another ? Thanks