Hi, Elusive Buttlerfly,
Thank you so much for posting your great question and also for sharing Roy's wonderful video with us. (I always look forward to hearing each song he uploads to YouTube! I subscribe to his channel.)
You will be happy to know that this duet is on a Glen Campbell album: "Still Within the Sound of My Voice", Glen's 43rd album released in 1987.
They sound great together, don't they? You might be interested in this additional information below.
Glen and Emmylou Harris duet on a non-Glen Campbell album:
An Emmylou Harris album re-released in 2004 by the name of "Blue Kentucky Girl". This re-release was issued on CD with two previously unreleased bonus tracks. One of the bonus tracks is a duet with Glen and Emmylou on the song "Cheatin' Is". Only this reissued album has the bonus track with Glen.
Emmylou is also a backup vocalist on Glen's album, "Letter to Home", Glen's 40th album released in 1984. She provides backup vocals on two songs:
"I'll Be Faithful to You" and "(Love Always) Letter to Home"
Emmylou Harris also performed with Glen at the Silk Cut Festival in the U.K. in April 1984. They performed "You Are My Sunshine": https://www.youtube.com/watch?v=B72ObGfDxGI
They might have performed another song together at this festival; does anyone on the forums know?