Emmylou Harris’ 10" vinyl LP (6 tracks) (released in 2023 as part of an Amoeba Records/Polly Parsons Kickstarter campaign that raised funds to release rediscovered Gram Parsons material) has a live track sung with Gram Parsons, recorded at a concert with the Fallen Angels in Boston, 1973.

Details here:

https://www.discogs.com/release/28955695-Emmylou-Harris-First-Album-Demos