Q. How should I address an envelope to a couple that is living together but not married?
A. This is very common and accepted now. In the days when the first etiquette guidelines were written, a couple never (gasp!) lived together before marriage. :)
In this case, state the woman's name first. Followed by the man's name on the line below. Then continue the address as usual.
Miss Jenna King
Mr. Paul Johnson
123 Grand Street
The same rule applies to a married couple when the woman chooses to not change her last name. Then I would suggest using Ms. instead of Miss.
Ms. Amy Brown
Mr. David Goldstein
New York, New York