I went thru my bookmarked videos and found a few more:

Guy Clark & Emmylou "I Don't Love You Much Do I?" two of the best ever, sharing a great song backed by Los Lobos - what a stage full of talent!
http://www.youtube.com/watch?v=RSUOaotjcSQ

Rodney Crowell & Rosanne Cash - "It's Such A Small World"
http://www.youtube.com/watch?v=cR0NmpL7N7A

I forgot about one of my faves - Ane Brun & Ron Sexsmith "Song No. 6" A very creative video, to be sure - more bluesy/jazzy than country/folk...
http://www.youtube.com/watch?v=jTSvZ_VlF5s

Anything from the BBC's Transatlantic Sessions will be an absolutely riveting, beautiful performance with wonderful collaborations between Celtic and Americana/Country artists - seek out anything with Eddi Reader, she's fantastic!

The late, great John Martyn & Kathy Mattea - "May You Never" WITH the brilliant Jerry Douglas on dobro! You'll recognize the song, as Eric Clapton had a pretty good sized hit with it...
http://www.youtube.com/watch?v=sBPTuAl2Qyk

Eddi Reader & Tim O'Brien "Back To Earth"
http://www.youtube.com/watch?v=Yw-aIDMB5Yw