How do you address an invitation to an unmarried couple living together?
When you're inviting a couple who lives together -- or a married couple with different last names, or an engaged couple for that matter -- list each name on a separate line on the outer envelope, alphabetically by last name, like so:
Mr. Richard Buxton
Ms. Ariel Dekovic
Put the names on the same line on the inner envelope, sans first names:
Mr. Buxton and Ms. Dekovic
Wedding Invitations + Wedding Stationery