Mr. & Mrs. John Smith

Mr John Smith & Ms. Mary Jones (if they are married and using different last names)

Mr. & Mrs. J. B. Smith. Mr J. Smith & Ms. M. Jones (if they are married and using different last names)

Dr. & Mrs. John Smith. If both are doctors: Dr. John Smith & Dr. Susan Smith. Dr. John Smith & Dr Mary Jones *as long as the couple is married, their names appear on the same line of an envelope, even if they do not use the same last names.

In the case they are co-habitating and not married the names appear on different lines. The use of Ms. is completely acceptable in even the most formal of situations. Use of Ms. may eliminate the misunderstanding that comes from the old practice of using Mrs. Mary Smith as a designation of a divorced woman (she no long uses her ex-husbands first name in her address)

---- Proper?

Mr. and Mrs. Herman Melville

But there's a very informal style that is in common use in the U.S. and that is acceptable for casual correspondence with people you know well enough to socialize with casually. It would not do for a wedding invitation or for anything else delivered formally (a graduation announcement, maybe a birth announcement), but it would do for a Christmas card or an invitation to a barbecue, assuming you were on familiar terms with the couple:

Herm and Sandy Melville