In a social situation, if two men are living together but are not married, a letter addressed to the two should place both names on separate lines of the envelope. However, if they are a same-sex married couple, their names are put on the same line.
"Mr." is used as the form of address for each man. For women who are a couple, very similar etiquette rules apply. The only difference is the form of address. For a married couple, either "Ms." or "Mrs." is used, depending upon their preferences. However, if one member of a same-sex couple self-identifies as the opposite gender and prefers the form of address associated with that gender, that title should be used.