I have a problem pretty exactly similar to this, with the exception that my chinchillas are female. I let them run together one time but the older female mounted the younger. I know this is just a dominance thing, but then the younger fur-slipped as the older one bit her. There wasn't exactly fur "flying", but still, I've never let them run together again and they are in separate cages. They're ok when I bring them nose-to-nose but anything else and the older one starts going crazy in my arms. I'm also too afraid to let them run in neutral territory after what happened last time, and I've switched their cages alot, too.