This is two of Pete Seegar's TV shows called "Rainbow Quest." The New Lost City Ramblers had Pete's brother in it, whose singing wasn't that good, but he was a marvelous autoharp player. That group was raw in the pieces they played, and Pete would play with them sometimes, too (he also plays alone at the start). YouTube has a copy of them playing "Soldier and the Lady" and a copy of them playing "Man of Constant Sorrow" from this show/DVD.
The Greenbriar Boys were more polished and more professional. They also played with Pete, who said that was the purpose of the show -- to learn each other's pieces. YouTube doesn't have any copies of the show, but there's other works of theirs around.
I loved the DVD, but the cats were a little worried that I harmonized.