Domestic Incident. People don't follow normal rules when they are thinking with hormones. Get two people with at odds hormones, not firing at the same time. and sparks start flying, and push turns to shove, and then it is Katie Bar The Door.
Storing firearms in a place where, two people have keys to that residence, and you had better know your partners mindset very very well. Even having the guns locked up, in a place where you argue with someone, is an invitation to trouble.