As a reference, we’ve got Neil Diamond’s “You Don’t Bring Me Flowers”, which he originally created to become a tv theme song, btw. Barbra Streisand did a version soon after, and some radio station dude created an artificial duet of the two singers and started playing it. it became so popular that the two singers got together and did a real duet, and the song hit #1 on the Billboard chart in December of 1978.
I am wondering if any other songs have gotten this treatment, with or without an actual duet following after. No parodies need apply.
Boz Scaggs and Rita Coolidge “dueted” on We’re All Alone. At least in the Baltimore area.
Nearly 30 years after it was a joke on the Simpsons, Cypress Hill performed “Insane in the Brain” with the London Symphony Orchestra.
Being Green, by Frank Sinatra and Kermit the Frog.
Were the songs artificially sang as a “duet” first?
Not sure. I think it originated as a stunt by a DJ in the Los Angeles market, and it may have been confined to the LA market. No idea if Sinatra and Henson ever hit the studio together.
Damn-I wish I could find a sample of that one.
Didn’t Natalie Cole do that with her father after he’d passed way? The song was Unforgettable.
And he had no say whatsoever in what she did with his song, so a real duet was never possible.
Oh okay, I got ya. I just read your OP carefully. Yeah it never became a real duet.
My local PBS station recently aired a 40th anniversary celebration of the Violent Femmes’ debut album, backed by an orchestra. I Tivo’ed it but haven’t had a chance to see or hear it yet. I’m wondering myself how well they pulled it off.
Which led to this. (The YouTube link is a still of the first frame.) Tammy Wynette was still alive at the time.
https://www.reddit.com/r/LiveFromNewYork/comments/1dhgcfg/unforgivable_a_hilarious_fake_ad_for_a/
Hilarious!
I am NOT dead!
Bear with me:
The (Nitty Gritty) Dirt Band recorded a song called, “Make a Little Magic,” which featured Nicolette Larson in a duet with singer Jeff Hanna. When they performed (read: lip synched) the song on Steve Martin’s comedy special All Commercials, Martin was seen on camera mouthing the Larson parts, in effect singing the duet with Hanna.
Fun fact #1 - the Dirt Band (as the Toot Uncommons) backed up Martin on King Tut.
Fun fact #2 - Jackson Browne was a founding member of the NGDB but immediately quit the group.
It may not be quite what you’re looking for, but it’s such a great partnership that I offer it here.
In 1971, Michael Parkinson was the host of a respected talk show on the BBC.
One evening two world-class violinists played together for the first time - Yehudi Menuhin (classical style) and Stephane Grappelli (jazz style.)
The result was pure class (and they went on to make recordings together!)
David Armand did a standup routine where he acted the lyrics to a recording of Natalie Imbruglia’s Torn. That became a real duet when she joined him onstage at a charity event. Granted, not exactly a singing duet, but close enough. 
Linkin Park-Jay Z. “Collision Course”
Originally it was going to be a technology mash-up of the existing recordings, but then they decided to collectively re-record the mash-up as more original performances, and they went on to perform them live at, among other places, Live 8 in Philadelphia.
Wow, that was amazing! I’m going to track down those recordings.