I’m trying to think of some male duet songs - either written specifically to be sung that way, or suitable for adaptation - any genre.
Any suggestions?
Outside of opera, the best one that comes to mind is Britten’s Canticle II, Abraham and Isaac - a duet between tenor and countertenor, and stunningly beautiful.
Broadway genre:
“The Confrontation” from Les Miserables, of course. Javert and Valjean in a gorgeous pissing contest.
“I’ll Cover You” from Rent has long been my favorite love song - sung by two men.
“I Can See it” from The Fantastiks
“On the Right Track” from Pippin
“All for the Best” from Godspell.
Geutary & Kelly, “'Swonderful,” An American in Paris
Tenor & Bass, “Suscepit Israel,” Pergolisi’s Magnificat
Judas & Jesus, “All For the Best,” Godspell
Bowie & Mercury, “Under Pressure”
Bunny & Fudd, “Das Nibelungenlied” (loose translation), What’s Opera Doc
Now that’s a creepy simulpost.
nah. It was a duet! 
Don’t Go Breakin’ My Heart, Elton John and Ru Paul
To All the Girls I’ve Loved Before, Willie Nelson and Julio Iglesias
Duel Duet, Brad Majors and Farley Flavors (Shock Treatment soundtrack)
Easy Lover – Phil Collins and Philip Bailey :o
Frank Sinatra had this album of duets, including songs with Luther Vandross, Julio Iglesias, Tony Bennett, Charles Aznavour, and Bono.
Pash
[ul]
[li]Al Green and Lyle Lovett, “Funny how Time Slips Away.”[/li][li]Jeff Beck and Eric Clapton, " 'Cause we Ended Up Lovers" on the “Secret Policemen’s Other Ball” soundtrack (instrumental, but male nonetheless.)[/li][li]A whole album of Ray Charles duets called “Genius Loves Company”, including James Taylor, B.B. King, Van Morrison, Willie Nelson, and Elton John.[/li][/ul]
Ebony and Ivory, Stevie Wonder and Paul McCartney
The Girl is Mine, Michael Jackson and Paul McCartney
“Hungerstrike” by Temple of the Dog (Eddie Vedder and Chris Cornell on vocals)
“Barroom Buddies” by (I think) Merle Haggard and Clint Eastwood
“Guys and Dolls” from the eponymous musical
“My Baby Said Yes” by Louis Jordan and Bing Crosby
“It’s Five O’Clock Somewhere” by Alan Jackson and Jimmy Buffett
“Dancin’ in the Street” with Mick Jagger and David Bowie
“If I Didn’t Have You” - Billy Crystal and John Goodman, Monsters, Inc. soundtrack
Isn’t Angel a trans woman?
Just Good Friends, Michael Jackson and Stevie Wonder.
Pancho and Lefty - Willie Nelson & Merle Haggard
For cheese factor there’s “To all the Girls I’ve Loved Before” (immortalized by Willie & Julio). Willie also did a duet of 'Luchenbach, Texas" with Waylon Jennings and a song called “Highwayman” with Johnny Cash and several other singers.
While he himself did not sing it as a duet, Cat Stevens’s Father & Son is a duet (he sang both parts himself).
No, just a drag afficianado. (He is first seen in the play dressed as a man.)
Almost forgot one of my favorite duets: Lennon & McCartney’s “Two of Us”.