Two male puppies can start fighting for a number of reasons; understanding them can help you determine the best course of action. Sometimes it's as simple as letting them have it out, while other times you should intervene to prevent an injury from occurring. When two puppies fight, it isn't always serious, but you should still have the tools to identify when it is. You and your two little guys will be better off for it.
Top Dog Position
When two males live together, one of them has to be the boss -- period. Dogs find comfort in social structure. In fact, a dog doesn't mind not being the alpha male, so long as he knows who the alpha male is. Puppies don't take long to learn who the top dog is, but it's not necessarily obvious from the beginning. So they may have it out while they're young on occasion to determine the alpha. Once one becomes the alpha, order is restored.
Puppies are sometimes territorial; they'll guard their favorite toys from even a sibling. If a puppy becomes attached to a toy and another puppy tries to take it, a little fight may break out. Territoriality is common for more than just toys, though. If a puppy develops a strong affinity for napping in a certain spot, for example, he may instigate a fight when another puppy takes it. If a sudden and short-lived fight occurs, then, it could just be the pups disagreeing over who has the right to a certain object or space.
Dogs play differently than humans do, and a lot of the time their playing looks like fighting. A serious fight is usually characterized by stiff muscles, prolonged posturing, hard bites and yelps, an inability to be interrupted by a simple command from the master, and swift, successive attacks. Play-fighting, on the other hand, moves at a slower pace. Your puppies may bound around energetically, roll and wrestle, take turns chasing and nip at each other without clamping down. While it looks a little like a fight, it's generally harmless. In fact, it's part of the dog's natural socialization process.
As puppies mature, their hormones kick in. If males aren't neutered, their hormones can make them aggressive. Puppies that aren't neutered are much more likely to be aggressive and attack each other, even if there are no potential mates to fight over. Even neutering just one puppy reduces the risk of hormone-induced fighting, as the intact puppy will not perceive him as a threat.