Then, there's always Marvin Gaye & Tammi Terrell, Aaron Neville & Linda Ronstadt, Richard Farina & Mimi Baez, Ian & Sylvia (real-life models for Mitch & Mickey in A Might Wind), Joe Cocker & Jennifer Warnes, and many many others ~ depends upon your interests/styles/capabilities, etc., and those of your audience.

You've had a bunch of nifty suggestions; when you decide what to do and start working it up, please come back and let us know!