when you and a friend fart at the same time and the smells combine
Dylan: did we just become fart brothers?
Ty: with our powers combined, we are fart brothers!
Two different dudes that have eaten the same ass. Somewhat similar to Eskimo brothers.
Person 1: "Bro I ate Niki's fart box the other day"
Person 2: "No way, I ate her ass last week! Does this make us Brazilian Fart Brothers?"