How do I teach 2 six month old Shih Tzus to get along

Both my daughter and I have puppies. I'm puppy sitting her pup for the summer. She has a female that has a very laid back personality, a true lap dog. On the other hand, my male pup is a ball of energy. Aggression arises when the female doesn't want to play or the male has a toy she wants or he goes in her kennel and gets her treat. I understand that some aggression will happen, but is there a way I can train them to tolerate each other better. Other than that, they play well together until one or the other gets tired. I don't dare leaved them out together alone. They'll play good for awhile then all hell breaks loose.

Any help is appreciated.