GULF COAST HIGHWAY by Nanci Griffith
is the ultimate duet, IMHO
http://www.coquet-shack.com/lyrics/Harris_Emmylou/Gulf_Coast_Highway_1844.php
http://www.chordie.com/chord.pere/www.roughstock.com/cowpie/cowpie-songs/g/griffith_nanci/gulf_coast_highway.crd
this has been done by Emmy Lou Harris and Willie Nelson.