call it them having a tooth fight
LOL - that's a BRILLIANT description. We call Murphy "Edward Scissorjaws" and it sounds precisely the way you describe.
Mara's head fits right into his mouth and he'll slabber and do the whole toothy thing but never bite. She, on the other hand, gives him pelters. She has been known to clamp down hard on his bottom lip and swings on it. It's credit to Murph that he doesn't retaliate and at least tries to play nicely - if a bit too 'big-ly'.
The only time she gets stressed is when he rolls on top of her and pins her somewhere (most often on the couch...)
We will NOT be taking them anywhere as civilised as Pets At Home until we are guaranteed that they will not cause carnage. Perhaps in about 2 years time...