Fighting at floor time.

My piggies, Tulip and Lilly, are sisters from the same litter and get along very well. Tulip is the boss but usually leaves Lilly alone when they are in their cage. But every time I put them out for floor time, they get into these verbal wars with each other. They seem like they are going to have a huge fight but Lilly backs down and Tulip puts her front paw on her and bites her ear!! They never do this any other time but at floor time. They are too busy fighting to play or even eat their lettuce. I don't want to seperate them because Lilly gets very fightened when she is alone. I feel so bad but I don't want them to get hurt. What can I do?