Most airlines state that they do not have such a thing as a married roster, even less so a"buddy" roster.
As in most of life, most things are negotiable but airlines are in my experience rather wary of these arrangements, especially if the people involved are not actually married in the eyes of the law.
Their reasoning (based on bitter experience) is that if they give person A and B a married roster, but a person C becomes involved with either of the former, one has a downroute disaster area, and their carefully made rosters are wrecked because person A no longer wants to be in the same part of the world as person B.
Go to them with real evidence of a permenant and stable co-habitive relationship that has withstood the strains of being rostered apart for long periods then you might find some favour.
Go to them with an ad-hoc request to be rostered with A N Other you are unlikely to get a sympathetic response and rightly so.