How can I get my puppies to get along?

We have a pekingese puppy who is 6 and a half months old. After taking her to a friends house and observing how much fun she had playing with their 2 puppies, we thought a companion would be a great idea. We bought another pekingese puppy who is 4 months old. Both of these puppies, individually, have very sweet personalities, but the new one is a bit more timid and quiet. The older puppy is very excited to play and she is probably double in size to the new one. She's older and plays a little more aggressivly than the new puppy likes, but has never offered to growl or nip the puppy. I've never seen her get angry. On the other hand when the little one has had enough she hides under a table or a small place growls and nips at the other one who will not get out of her face, because she is still in play mode. I'm not sure what to do. It is more peaceful here when I just keep them separated, but of course that is not the solution. How can I get the older one to respect the little ones boundries when she has let her know she's had enough? Should I just let her antagonize the little one and let the little one nip and growl? if I stay out of it will they just work it out? I feel sorry for both of them. They are both females and the older one was just spayed.