J.B.
I provide childcare to a set of 18mo twin boys. They have bitten eachother on occasion. I have been doing alot of reading about this and according to some of the things I've been reading- 99% of the time, biting is done out of frusteration or anger. The other 1% is done when a child is playing. Your girls may or may not be able to communicate with one another about how their feeling or what they want or need, so they will bite out of frusteration or anger. Children quickly learn that biting causes a reaction, which is what they are looking for. My advice would be to try to work on having them verbally communicate what it is they need or want. You may have to follow them around for a few days to see what sets them off and causes the biting. But, by "shadowing" them, (as it's called) you may be able to see the "triggers" (what caused the child to bite) and stop a biting incident before it happens.