What kind of hamsters are they? If they are syrians they need to be separated.

They may be fighting over a number of things, space, food, etc. Make sure that they have all of these things in good supply. I know this may sound silly, but are you 100% sure they are both female? Males and females are generally hostile around each other, and if that is the case they need to be separated right away before you end up with a litter.

If you can't figure out why they are fighting but they continue to harm each other, really they have to be separated. Wounds become infected easily and it's hard to treat an animal as small and as delicate as a hamster.

I hope they stop fighting soon!