Other than what was already mentioned, correcting with a strong voice, I don't know. I would always keep an eye on them and not let them play together unsupervised.
As I type this, I literally have a 'fight' happening on the couch beside me. I got Cooper when she was 4-5 months from her previous home, where they had gotten her at 5 weeks!! (so they say, I believe earlier.)
Anyway, I think her being seperated from her littermates too early has made the difference.
She doesn't really know how to play.
I really had to watch her with Maddie b/c she was always
SO rough. Thier playing always led to growling, biting, and Maddie screaming. It has been a long road, but we they play much better now. I still will not leave them out together while I am not home.
Hope someone has more helpful suggestions.