IF they are both does it will be their teenage hormones kicking in. Depending on how bad they are they may be ok to stay together for a few more weeks until they can be spayed or they may need to be seperated until they can be spayed at 4-6 months and then rebonded.
As Jane pointed out they could be a boy and a girl, buns are hard to sex when young and pet shops arent always great at it. Have they been vaccs yet? If not I would book them in for their first vaccs and get the vet to sex them at the same time