Some chins get along and some don't. They were never well socialized so that is probably why she is so scared. Take your time with her, give her treats. Be patient. See if you can scratch her a little. Healthy treats are the way to getting a chin over being so frightened. If they were not socialized then this is why she is behaving like this. And the mounting is breeding behavior. If you have a male and female they MAY have babies, I hope your prepared.

What you can do is put two food dishes in the cage and two water bottles in the cage, that might solve the problem.