Dogs like to play rough sometimes, and while it can seem like they are really going to hurt one another, it usually turns out that they really are playing in a dog sort of way. This is a perfectly normal mode of dogs getting acquainted and learning to communicate with one another. It does depend a lot on the breeds of the dogs too. If you have a pit bull and a Lab playing rough, the pit bull may not know when to stop, and you may wind up with an injured Lab.
Some dog “experts” say that interrupting dogs when they are playing like this is inappropriate, but if you see that things are getting out of hand, you may just save yourself a high vet bill. Dogs will “mock fight” with snarling and soft biting moves, that seem pretty realistic at times. This is how they get to know one another, and is probably how they determine to a point who is going to be the pack leader.
They will place their chin over the back of the other dog, mock bite at the legs, and attempt to mount the other dog to show dominance. That is acceptable behavior if it does not escalate any further than that. Most dogs will “play” like this until they get tired and quit, and most of them will eventually settle into a friendly relationship with each other that will last a lifetime. It is a good idea to always monitor these episodes, especially in the early stages of the relationships, just to be sure that things are still platonic.
Of course, you must establish yourself as the “lead” or alpha dog, or they may not pay any attention to you. It is not a good idea to attempt to break up a real dog fight. Different dogs have different natures of aggressiveness, and different breeds vary also. If your dogs look to you as the pack leader (because you feed and take care of them) they will succumb to your commands, even if you have to get forceful with your voice. If the dogs can play in this manner without causing any permanent damage, it will benefit them in the long run by giving them a good amount of exercise.
Aand forming a solid relationship between them, that will have them being the best of friends for their lifetimes.