Q: Why does my ferret attack my other ferret when I won’t let them out of their cage?
A: What you describe with your ferrets is what we call redirected aggression. Basically, in human terms, the ferret gets frustrated that you won’t open the cage but he can’t get to you. So the aggression turns on the closest object, in this case another ferret.
One way to try to deal with this is to get the ferrets interested in something besides you when you are around but unable to let them out. This could be a new toy, hidden treats or any other enrichment idea. You can also reduce the times that trigger the behavior by paying attention to when it does and doesn’t happen.