Sometimes dogs feel their way around to see what their state in the household is. Sometimes they try to be top dog (pun intended) but as we know they cannot. Overtime, we humans assert our authority as the "alpha" and our pups know that. In a show of submission, they roll on their backs exposing their belly - a very vulnerable state. Here we have Ming and Duncan rolled over. We'd show Pink also but she has a stubborn streak and quite frankly, the camera shutter cannot keep up with her.
While the puppies are used to each other in nearly four months of life, they sometimes play and attempt to assert their dominance. Here is another scenario in which Pink is happily owning the dog bed. When Duncan comes in, they seem to want to play but also are positioning on owning the bed. It looks more fierce than it was. Ah the day in the life of a puppy.