Trying to put myself as the man in this situation. I'm kind of scared, but also a little aroused.

Here is how I see it playing out:

They start fighting.

My girl wins. Sweet she is going to get a foot rub and the fucking of a lifetime that night. 

My girl loses. I now have to shoot this girl in both feet with my sidearm. Drag my girl to safety and call the paramedics.

Neither girl wins. They stop midfight. Start negotiating terms for a 3 way marriage as a lady's compromise.