When you say biting what do you mean? Like there flipping each other over on their backs or actually lunging at each other and squealing.
Rats DO wrestle....mine are in my bedroom and I've been woken up plenty of times to them wrestling in their cage. Usually there's only a bit of squeaking. If mine are being really annoying and making alot of noise (ex rattling the cage) I just go "hey! Knock it off NOW!" that seems to work and they just go back to running around.
If there actually fighting there will probably be blood or lots and lots of squeaking from the weaker rat.
If you just threw them in the cage together that is probably the reason for them fighting. You need to separate them if this is the case and gradually introduce them.