And of course, a playlist like this can't be complete without this one:

Elton John and Billy Joel - Goodbye Yellow Brick Road

Paul Simon and George Harrison

There are quite a few collaborations I can think of when considering this question. I would like to start with a recent one, Gorillaz (Damon Albarn) and drum master, Tony Allen in the song How far. James Blake and Rosalía singing Barefoot in the park. The well known band from the late 90s and the begging of this century made an entire collaborations album called To All New Arrivals, which is worth mentioning. Last but not least, Poliça and Justin Vernon (Bon Iver) collaborated for the song Tiff as part of the Deluxe Version of Shulamith

Keith Richards & Norah Jones ~ Love Hurts, live 2004 HQ:

Mark Knopfler & Emmylou Harris - If This Is Goodbye:

Deftones ft Maynard James - Passenger

Gonna throw you a curveball here. It's a collab and a cover.

Don Henley and Vince Gill singing Sacrifice by Elton John.

I am not a huge country music fan so was never really into Vince Gill but he is awesome with the Eagles.

I Don't Want To Talk About It (from One Night Only! Rod Stewart Live at Royal Albert Hall) - featuring Amy Bell:

U2, Leonard Cohen - Tower Of Song:

Now this is one melodious harmony

You can post a link to that song here let me listen to their voices interwine.