Given the above responses to my first post in this thread, I will now shame-facedly slink out of this thread. :smack:
Not yet mentioned: Gregg Rollie and then-new-to-the-band Steve Perry duetting on Journey’s “Feeling That Way”. Talk about a changing-of-the-guard moment!
If a TV performance counts, once on Saturday Night Live, Paul Simon and Willie Nelson dueted on “Graceland”. Boy, I could listen to Willie duet with anyone.
Those who don’t know already don’t care, but the Grateful Dead in “Jackstraw” have two different singers with two different perspectives (one of which ends up killing the other).
Now that someone has mentioned Saturday Night Live, one of my favourite duet performances was Joe Cocker and John Belushi as Joe Cocker. Hilarious.
Antonio Banderas and Eddie Murphy did “Livin’ la Vida Loca” for Shreck 2.
On The Bridge, he sings a song called “Baby Grand,” with Ray Charles. I mentioned it above.
Don’t forget Miss Piggy…who, of course, is really a man. 
If we’re gonna include bands with two male lead singers, I’ll toss in Roger Daltrey & Pete Townshend of the Who, most notably on “Baba O’Reilly”.
Willie Nelson did a duet with Kid Rock on his “Great Divide” album from 2002. The same album also has duets with Rob Thomas and Brian McKnight.
Ozzy and Lemmy did I Ain’t No Nice Guy.
Bruce Dickinson and Rob Halford did The One You Love to Hate
Gary Moore and Phil Lynott did Out in the Fields and Miltary Man
Alice Cooper and Twisted Sister did one, but off the top of my head I can’t think of it.
I know I’ve got more metal songs as well.
Otis Redding & Wilson Pickett: “You Left the Water Running” Gold!
Elton John & Neil Sedaka: Bad Blood.
Eddie Vedder and Chris Cornell
“Hunger Strike” by Temple of the Dog.
There are a lot of male to male duets in the context of a band. For instance, many Beatles songs were duets between John and Paul.
‘Wasted years’, ‘Gloria’ - John Lee Hooker / Van Morrison.
I don’t know if we’re considering Broadway shows, but in Les Miserables the song “The Confrontation” has Valjean & Javert singing different lines at the same time to the same tune.
If we include Broadway, Leo Bloom and Max Bialystock in “The Producers” sing several songs together. As do other actors. “Keep It Gay,” “I Wanna Be A Producer,” and “Springtie for Hitler” are just a few.
I have “I Wanna Be A Producer” off of the Broadway: The American Musical CD collection and it’s a HUGE production number, but it’s just Matthew Broderick singing with a chorus doing backup. Is there another time that’s sung in the show where they both sing?
Air supply - All out of love
Anything by The Zombies.
mmm
Sinatra’s Duets came out when I was working on the radio. The DJ and I constantly made fun of “I’ve Got You under My Skin” (Old Blue Eyes and Bono), putting it on our list of “Songs That Suck.”
A well-known (and batshit crazy) man-about-town of a certain ethno-religious persuasion was a frequent guest on the show. When he used the line on me, I said “Ziggy! I thought you were Jewish!”