My most famous duet is Cinderella Rockefella. It was made popular by Esther and Abi Ofarim back in 1968. It was a hit in every foreign country, except Japan and other parts of Asia. It never hit big in the US, but it was performed on The Smothers Brothers Comedy Hour several times. It’s a novelty tune and might be considered to be in the English Busker-Skiffel group style. There are several videos of it on YouTube including a performance by The Carpenters that I didn't even know existed! Cinderella Rockefella was most recently used in the film, Walk On Water.
I wrote and arranged a few different songs as duets for television shows that I worked on back in the 60's and 70's. One of the things we explored was the notion of quodlibets... an arrangement that is two songs sung together at the same time.
I arranged Words, by the Bee-Gees with Words of Love, by the Mamas and Papas for Andy Williams and Cass Elliot to sing together for her first television appearance without the Mamas and Papas. The show was called the H. Andy Williams Kaleidoscope Company.
My most popular combo was aired on The Summer Brothers Smothers Show. Glenn Campbell was the host and Allen Blye and I worked up a duet with him singing By the Time I Get to Phoenix, and Nancy Sinatra singing, I Say a Little Prayer. Glenn eventually recorded this arrangement of I Say a Little Prayer with Anne Murray on one of his albums.