Hello, I have recently acquired 2 Egyptian spiny mice. They are both female and have lived together happily for the past 2 years. A few weeks ago I found blood in the cage and saw that one of the mice had small nicks on her tail. I've never seen them fight so I assumed it was over grooming. I added more toys, and made sure there was two of everything food, water, house and wheel in case it was fighting. This seemed to help until today. That same mouse now has a relatively large wound on the middle of it's back. It bled but has now stopped. She is also missing hair from both sides of her back. I read online that Egyptian Spiny mice can self-flay. I'm not sure if that's the case. They seem to be getting along fine. They always share a house and wheel event though there is two available. They do chase each other but I've never seen them bite. I know they are very social so i'm reluctant to separate them, I don't want to cause her more stress if it is in fact self inflicted.

Any advice is welcome!

I do not have any experience with Spiny Mice. However, based on what you have described, I would think one is being bullied by the other. I recommend separating them temporary and see if the wounds continue.