Johnny Cash are certainly the most famous husband and wife singing duo in history. Their story is so beloved it was made into an Oscar winning film.
Second place would probably go to another couple whose story was made into a very different sort of film. That would be Ike and Tina Turner, whose abusive relationship was captured in What’s Love Got to Do With It.

I recently read a fascinating profile of Kris Kristofferson (by Ethan Hawke, of all people) in Rolling Stone magazine. Hawke painted Kristofferson as a modern-day renaissance man and an unabashed sentimentalist and political liberal who is nonetheless about as “manly” a man as can be.