I used to say that Miss Manners was the best columnist the Washington Post has, but now that David Froomkin is gone there isn't even any competition. And she takes on addressing same-sex couples:
Besides, updating old customs for new times happens to be her favorite part of this job. And there are formal forms other than Mr. and Mrs. from which to choose. In fact, that is the most awkward, although the most familiar, form since it does require choosing one given name, which unnamed halves are increasingly finding off-putting.
The plural of Mrs. is Mesdames and the plural of Mr. is Messrs. So a married female couple with the same surname would be Mesdames Jenna and Aurora Acorn, and a married male couple would be the Messrs. Jackson and Hal Thornton.
Then there is the two-line form, each name with its own title. This is used when an address is shared by unrelated people or by family members, such as siblings, but also to address married couples if the wife has a title such as Dr., Senator, Judge or Dame, or, you should note, if her surname is different from her husband's.
I don't know the next time I'll be writing to an official "Dame," but I'll keep in mind that married Dames get their name on a separate line of the envelope.