What are some bands where any one concert, or album, is likely to feature two or more people sharing lead-vocalist duties?
Two examples that come to mind:
Styx (at least, in their heyday; can’t say what’s going on with them now) would oft feature Tommy Shaw doing lead vocals on some songs (Blue Collar Man, Foolin’ Yourself); James Young on one or two others A(Miss America, Snowblind); and Dennis DeYoung handling the lion’s share.
The Eagles seem to divvy up the lead vocal duties fairly evenly. A cursory sampling of some of their top hits has Glenn Frey singing Take It Easy, Don Henley singing Hotel California; undoubtedly Joe Walsh has taken the lead a time or two as well, though I don’t feel like researching The Eagles’ entire catalogue right now to be certain.
The Grateful Dead. Originally Jerry Garcia, Bob Weir and Pigpen, with Phil Lesh on backing vocals. Later, Donna Godchaux, then Brent Mydland, Vince Welnick and Bruce Hornsby all sung whilst playing keyboards.
Also damn, being a keyboardist for the Dead was about as good a recipe for long life as being a drummer for Spinal Tap…
Just off the top of my head: The Beach Boys: Brian Wilson, Mike Love, and/or Carl Wilson on many of their best known songs, but the other band members have also contributed lead vocals. Fleetwood Mac: Stevie Nicks, Lindsay Buckingham, and Christine McVie in their most famous lineup The Moody Blues: Justin Hayward, John Lodge, Ray Thomas, and on earlier records Mike Pinder The Bangles: All four members sang lead at least occasionally XTC: Andy Partridge sang (and wrote) the majority, but Colin Moulding contributed at least a couple songs to each album. The Alan Parsons Project: Used many different lead vocalists on each album. Eric Woolfson and Lenny Zakatek are probably the most prolific and most recognizable, but there were many others. The Kinks: Ray Davies was the lead singer, but brother Dave got at least one or two lead vocals on most of their albums. Trip Shakespeare (a personal favorite): Matt Wilson, Dan Wilson, and John Munson shared lead vocal duties
And then there are the “supergroups” formed by multiple singers, such as The Traveling Wilburys, The Highwaymen, Lost Dogs, The Thorns, etc.
Crosby, Stills, Nash and Young, and all the various spin-offs, as well as all the bands the members came from (The Byrds, The Hollies, Buffalo Springfield).
The Dubliners had several vocalists in every lineup. The original four - Ronnie Drew, Luke Kelly, Ciaron Bourke and Barney McKenna - all sang, and later so did Bobby Lynch, Jim McCann, Sean Cannon, Paddy Reilly, and Patsy Watchorn. Same with many folk groups.
Spirit – Jay Ferguson, Randy California, and Mark Andes all did lead vocals.
The Bonzo Dog Band – Vivian Stanshall, Neil Innes, and (sort of) Roger Spear sang for the group.
Peter Paul and Mary
The Who. While Roger Daltrey usually did lead vocals, John Entwistle sang his own songs, and Pete Townsend would occasionally join in. “Quadraphrenia” also had vocals by Keith Moon.
Jefferson Airplane – Paul Kanter, Marty Balin, and Grace Slick all sang lead at various points.
The Righteous Brothers. Billy Medley did most of the singing on You’ve Lost That Lovin’ Feelin’ with Bob Hatfield duetting in parts, but Bob Hatfield sang Unchained Melody as a solo.
Deep Purple Mark III used to split vocal duties between Glenn Hughes and David Coverdale. And the Faces had both Rod Stewart and the late, great Ronnie Lane.