2 females fighting

Question
We have had 2 female Guinea pig for about 3 months now and they have stayed in the same cage.  The problem we used to have was the tri--colored one would push the black one around in the cage. Now the black one is bitting the tri--colored one on the rump and causing the tri--colored one to squeal.  We now have to constently keep one out of the cage. What should we do?

Answer
Either the cage can be too small and the guinea pigs could be fighting for territory or they are just not going to get along at all. Generally when you have two guinea pigs the cage should be 30"x 50".  If the cage seems big enough for the two you may just be seeing dominance issues. That will eventually end once they get older as long as it doesn't get out of hand. If she starts drawing blood then you may need to seperate for good.
Since they are maturing they will get a bit more aggressive for a little while. Some people add more chew toys into the cage to help with aggression and to keep the guinea pigs mind off of the other one for a while.