I think everyone is right when they say it depends on how aggressive each dog is. But also when ever you have two dogs you should be careful when they are eating dinner and chewing their bones because threy might get possesive of the items.
Thats how my two males usally get into a fight. or if they start playing too ruff. We usally have a fight once every two months. they usally have a couple punchers each. but it doesn't bother them that much. they make up and become best friends again!