Sounds like the infamous female-female competition building up. Rough play can easily escalate into a serious fight. Do not leave the two together unsupervised. There are several threads on this topic if you type it in the search button. Consider hiring a trainer to come to your home and determine the best plan of action. But interview several to make sure you like their philosophies.
PS: buying two of the same toys will not solve things. They always want what the other has.
Deja, GSD, sable WL, female, born 11 / 2013
Cam, Rough Collie, sable, male, born 8/2015