Gillian Welch: I Want To Sing That Rock And Roll
[purchase]
The David Rawlings Machine: Elvis Presley Blues
[live bootleg]
Glen Phillips: Revelator
[purchase]
Co-writer, producer, and constant companion David Rawlings may be willing to let Gillian Welch have the billing, but those in the know understand that the act known as "Gillian Welch" has been a true partnership since its inception in the early nineties. As such, most Gillian Welch songs are duets by definition.
As proof, we need only note that pretty much every cover of a Gillian Welch song I have ever collected retains Rawlings' subtle, unbilled but everpresent harmonies and intertwined second guitar part as if they were inherent in the tune itself. For example, though Glen Phillips is billed solo, the duet harmony leading into the chorus on his cover of Revelator is neither a surprise nor a fluke; similarly, you'll hear the exact same harmonies on Chris Pureka's cover of Everything is Free that you will on the Welch/Rawlings original. I also note that the two perform together on occasion as The David Rawlings Machine: when they do, though their material tends towards coversong, the guitar interplay and harmonies remain static; the only difference is that Rawlings is more prone to sing the verses, as he does in the bootlegged "Gillian Welch cover" above.
Here's a typical set, with originals under each name and a cover to split the difference; for more on the partnership, check out this longer treatise on Gillian Welch I wrote back in January '08.