I think it depends on a few factors, including whether they knew each other before you brought them home, whether there's a large size difference between them etc.
They may have settled with you enough to do stuff with gay abandon, rather than an eye out for other things.
It could be them deciding on the pecking order
It could be deflected aggression caused by something outside, though that seems unlikely
I tend to go on the theory that if there's no blood, no tufts of fur, and the vocalisations aren't like screaming, then they are playing or at least not trying to cause real harm.
Boycat (may he rest well over the Rainbow Bridge) and Girlcat would be completely silent when they played, both exceptionally quiet cats and Girlcat still is now. It would look really aggressive, until you realised that there were no claws, they would only partly close their jaws and only on areas with thicker fur, and there were never any tufts of fur afterwards. They would take turns, then curl up on my bed together when they were worn out.
Tobias is very vocal and announces to the world every single thing he does. He yells, makes growling noises (not real growls, sort of like puppy growls), makes a noise like a crow on 40 fags a day sometimes.
I had an interesting few months when he came home while Girlcat put him firmly in his place - she was the incumbent, older than him & most importantly female so she made sure he knew his place. But the one time she actually hurt him was definitely a mistake on her part, and SHE ran away crying (she'd caught him on his face with a claw tip, & there was the teensiest spot of blood). The still play fight now, her silent and him grunting & growling away in exactly the same tone he uses when he is killing a toy.