Pop culture gadabout Bill Sherman (that's the name of his fine weblog) calls my attention to the "other side" in the matter of The Youngbloods not appearing on Johnny Carson's show. An article about the group can be found here and I'll quote the relevant line…
Although [group leader Jesse Colin] Young at times would get annoyed with record companies that did not promote his albums enough and although he walked out on the Tonight Show when Johnny Carson went back on a promise to let the Youngbloods perform two songs, the singer enjoyed a fairly idyllic life in West Marin.
It's always possible there was a misunderstanding but I find it hard to believe The Tonight Show promised two numbers to a musical act of such limited stardom. With most performers, even one wasn't guaranteed if the show was running long. At best, they'd tell an act to rehearse a second song just in case Johnny decided the first had left the audience eager for more.