
How old are they? Have they both been house trained fully? How does you girl get on with other bitches?
My grandparents kept boxers together for years and never had a problem with any of them, they had both dogs and bitches, all getting along fine most of the time.
I would say phone the breeder up and talk about it. The breeder might be more then happy to let you have the other girl as they feel you are responsible boxer owners....