Male and female dogs are less likely to fight. This doesn't mean that they will never fight, but they are less likely to fight.
If you are having problems with this, it is probably because one of the dogs is trying to dominate the other in order to become the leader. It is up to you to decide who is the leader. Some ways of doing this is to always feed one dog first, to let one dog have the better sleeping spot, etc.
I have a two year old male and now have a 10 week old female. Although they get along just fine now, I am taking precautions to ensure that they don't fight. Cash isn't a fighter, but still. I always feed him first. I greet him first when I get home. He gets the better spot on the bed (my spot is the best
).
The people who said they will not fight is not wrong, it just doesn't apply to everyone. Some dogs have more dominant personalities than others and it is a good idea to make sure that one of your dogs has a dominant personality and the other is submissive.
Good luck.