Polymandry

/ˈpälēˌmandrē,ˌpälēˈmandrē/
A play on the word "polyandry" when a wife takes more than one husband.

Poly-man-dry is the occurrence when you find out your best friend has another man friend on the side.
Man1: "Hey, do you want to watch the Superbowl this weekend?"
Man2: " Sorry, I'm hanging with my other man friend"

Carter is practicing polymandry, he has another best friend on the side.