Welcome to Yorkie Talk. I would start with having the girls spayed. It isn't a behavior cure all, but I would do it for long term health reasons and the chance it could help with the fighting.
Is the fighting always triggered by specific things like guests in the house? If you can identify the triggers, that is half of the battle. Separate the girls when visitors come, or leash them and work on training to greet visitors. Training tips can be found on YouTube. I prefer positive reinforcement training methods.
If the girls are fighting when they are not supervised, I would separate them for safety.
__________________ Kristin, Max and Teddy  |