McCaslin and Ringer "Bramble and the Rose" album - if you can find it.

any Jody Stecher and Kate Brislin but especially "Heart Songs" (all Utah Phillips' older stuff)

any Tom Russell that he has Katy Moffat with him on - esp. "The Long Way Around."

some of Skip Gorman's cowboy records where he has Connie Dover helping out.

but Ian and Sylvia will forever define male/female duet singing - for people my age, anyway.