Eric Clapton & Mark Knopfler - Wonderful Tonight
http://www.youtube.com/watch?v=o8ipQl9pywg
Yep, their playing styles really fit together well. There's a clip of Layla on YouTube as well - the slow "Unplugged" version, with Clapton on the acoustic guitar and Knopfler on the electric. Mark's collaborations with Chet Atkins are very nice too, if you like pure instrumental work.
Chet was the technical master of the 6-string, but Knopfler's playing has an almost vocal quality to it.