you posed the question over a year ago, but I just joined the SDMB, so this answer may have been overtaken by events. at any rate, I think you’ll find the Tina Turner and Bryan Adams duet on “It’s Only Love (and that’s all)” to be an excellent rock/pop song, not smarmy or syrupy at all.
“Don’t Go Breaking My Heart” - Elton John and Kiki Dee
P!nk and Nate Ruess, Just Give Me A Reason: P!nk - Just Give Me A Reason ft. Nate Ruess - YouTube
Here are a couple more: “Bruises” - Train and Ashley Monroe, to mention a fairly current song; “Islands in the Stream” - Kenny Rogers and Dolly Parton, to go back about 25 years or so…
“You & I” by Wilco is a fantastic duet between Jeff Tweedy and Feist.
Marvin Gaye and Tammi Terrell did Ain’t No Mountain High Enough and Ain’t Nothing Like the Real Thing.
Me Quedaré Solo by the Spanish 90s pop group Amistades Peligrosas. Good pop song, evenly matched between the male and female singers, with classical instruments (flute, tympani, etc.) in the mix.
Someone mentioned Peter, Paul and Mary upthread, and I’ll echo that. “Puff the Magic Dragon” is a favorite of mine. 
Well, I’ll probably catch hell for this, but Close My Eyes Forever by Lita Ford w/ Ozzy Osbourne.
I saw them do this together live, just him with a keyboard and her. One of the greatest live music experiences I’ve ever had.