Al I can think of is Paul Simon and Paul McCartney.I think Dave Grohl might hold the record for most appearances by a musician
Nirvana - 2x
Tom Petty - 1x
Paul McCartney (with surviving members of Nirvana) - 1x
Them Crooked Vultures - 1x
Foo Fighters - 9x (including this upcoming appearance)
14 times in total