Colette came up big with John Prine's album - which is a total blast. Does a 'pure' duet need to be call & response, or whatever that structure seems to be or is it simply two singers trading verses and harmonizing on the chorus? At any rate, here are a few more that popped into my head:

Frank & Nancy Sinatra - "Something Stupid"
Sonny & Cher - "I Got You, Babe" and numerous others
Sinead O'Connor & Shane MacGowan - "Haunted"
Peter Gabriel & Kate Bush - "Don't Give Up"
John Travolta & Olivia Newton-John - "You're The One That I Want"
Elton John & Kiki Dee - "Don't Go Breakin' My Heart"
Jerry Jeff Walker & Carole King - "I'll Be Your San Antone Rose"
Dan Fogelberg & Emmylou Harris - "Only The Heart May Know"

Didn't Emmylou and Gram do some duets or was it more lead & harmony? Linda Ronstadt & Aaron Neville did a few on her Cry Like A Rainstorm... album. Some songs by Iron & Wine get close, but probably not quite duets - tho brother & sister Sam and Sarah Beam blend wonderfully together, check 'em out on Austin City Limits with Aimee Mann from last fall. Keely Smith & Louis Prima. Dean Martin did a few that are escaping me at the moment. Seems like Dolly Parton did a mess o' duets over the years, too - that one with Kenny Rogers.

http://www.cmt.com/shows/...sode_featured_copy.jhtml
A quick search yielded another 100, but you'll have to sift thru on your own to find the male/female ones (I'd forgotten about the Bobbi Gentry & Glen Campbell album...had that as a kid.) This turned out to be a really fun question, thanks Ron!