My shih tzu likes to chew on my mini pin. how do i stop this?

Mini pin melody is 6 and don't really seam to mind but some time the puppy rosie bites to hard and I'm scared the melody is going to hurt rosie she's only 6 months. Mostly it play but there is times with growming hair up and I have to pull them apart. What would you do. Melody has hurt rosies eye once while they were fighting.