Everyone, of course, must think of the obvious: Dylan and Lightfoot.
I kind of fancy the combination of Lightfoot and Carole King.
Or Jann Arden and Gord.
Gord and either Joni Mitchell or Joan Baez would be a classic.
G.L. and the late Eva Cassidy, who did a marvelous cover of EMR!
Keeping in the vein of the more obscure combos (like Iglesias & Nelson) what about
: G.L. and Pavarotti! (After all, Gord has been described as a former Opera Hall singer in some accounts of his career!)