Not sure if Temple of the Dog counts, since they were a one-off, but they featured both Eddie Vedder and Chris Cornell on vocals.
His recent passing reminded me: While written by Ric Ocasek (RIP), the track “Drive” was sung by bassist Benjamin Orr
First two sentences of the OP.
Humbly noted.
Eric Bloom is the primary lead singer of Blue Oyster Cult, but Buck Dharma also sings lead on some tracks–including “Don’t Fear the Reaper” and “Burnin’ for You,” their most popular songs.
I was surprised to find out that the Bangles actually split lead singing duties between them, but the record company kept pushing the Susanna Hoffs songs as singles.
The first few Alan Parsons Project songs I became aware of had Eric Woolfson on lead vocals, so it was a bit of a surprise when I found out how many different lead vocalists they used.
For Steely Dan’s first tour (and a couple songs off “Can’t By a Thrill”) David Palmer took on the vocal chores before it was decided that Donald Fagen’s voice was preferred.
Saw an old video of “Do It Again” and saw some ugly dude singing on stage, and I’m like - that’s not Fagen!
heh - did John Oates ever sing lead?
More often than I realized, according to this list in his Wikipedia entry.
I’m aware of the fact that the Eagles had more than one singer, but I’ll be damned if I can tell them apart.
For over a decade I had no idea that guitarist Graham Coxon contributed lead vocals to a number of Blur’s songs, including my very favourite “Coffee and TV”. I had simply assumed that Damon Albarn sang lead on everything. I discovered my error only some time in the last couple of years, when reading the Wikipedia article for the band or one of their albums.
True, but the Walk like an Egyptian video, arguably their best-known song, made it very clear that each of them had singing (or whistling) duties.
I can probably tell Frey apart from Henley with about 80% accuracy.
But I’ve gotta admit I frequently can’t tell who is singing lead between John and Paul in early Beatles tracks. 
The Goo Goo Dolls. Every single hit on the radio has had Johnny Rzeznik singing the vocals, so I was disappointed when I bought A Boy Named Goo and encountered a whole bunch of songs featuring a different singer that, in my opinion, sounded completely different and not as good.
Another one I didn’t realise for a long time is ZZ Top - Dusty Hill does the lead on a few songs, probably most famously Tush.
Those damn CSNY kids…
I can usually pick out when it’s Nash (lead in “Our House”, with barely detectable Brit accent) or Young (well, 'cuz Neil), but Crosby and Stills are trickier because they slightly blend together a bit more, I find.
A fair number of the earliest Beatles songs (From Me To You, She Loves You, I Want to Hold Your Hand, and so forth) feature both John and Paul, singing in two part harmony. It’s arguable which of them should be called “lead” vocalist on tracks like that.
Singularly noteworthy example: Dez Dickerson, Lisa Coleman, and Jill Jones have some lead vocals on “1999,” which was, IIRC, completely unheard of to that point on a Prince album.
(Dez also gets a guitar solo on “Little Red Corvette.”)
Frey had the more “ordinary” voice. Same with Grateful Dead songs, the voice with almost no vibrato or diaphragm stress is Jerry.
We were such Beatles fans as kids and would lipsync (using tennis rackets as guitars) and do each Beatle’s mannerisms. But damned if we could tell when The Kid Being Paul should sing, or back off the mophead microphone and let Jimmie Trying To Be John should sing.
We weren’t the only ones, and the record company actually printed who was singing on each track on the back of the LP covers. Didn’t help us at that age, though. All we could afford were the 45s…
I had no idea Colin Blunstone of the Zombies was one of those.
He and Rod Argent had just reunited and I ran up to Milwaukee where they played for hours: early oldies from their garage band days, Zombies stuff, solo work, then Manfred Mann and Argent songs. Then we all looked at each other in surprise as they launched into some Alan Parsons… A history of R&B-Rockabilly-Prog-Moody-Rock for free*…
*Summerfest, got in “free with the donation of a slightly used children’s book” and saw them on one of the free stages.
‘You’ve Got That Lovin’ Feeling’ is the only one I could name without looking it up…
Or George, I’d add.