Do you have something like soy milk. Rats like that, well at least mine do. Especially when you mix together soy milk and baby food (dry baby cereal usually mixed with water or milk). Like Lindsay said it's easier to give them something like that because they can't grab and run off, they have to stay there and share.
Keep in mind that unless things get really rough, squeeking, mounting and chasing each other around is normal. I let mine meet for a week together before I put them together.