One Band, Two Or More Lead Singers

[Mofo Rising]

The new Beatles version of the game Rock Band will feature three microphones, so people can harmonize on the songs. I have enough problems butchering the vocals of a song with one vocalist, so I can imagine how awful it may sound to listen to people harmonize. Then again, I know a lot of people who can now sing better because of that game. (That includes me.)

The only band I can name off the top of my head that uses two singers is Guster, a somewhat indie band. They have one album I really like, and all the rest I don't care for.

[Allhallowsday]

JEFFERSON AIRPLANE had both GRACE SLICK and MARTY BALIN:   

Didn't Jorma sing as well?

Hmmm, I used to know AIRPLANE details better way back when than I do now... I still love them and have several of their albums, but I have to say, don't know, REV.  Maybe he sang for HOT TUNA?    

He definitely sang for Hot Tuna... I only have Surrealistic Pillow, and I always assumed it was him on "Coming Back to Me", but it's probably Balin.  I'm less of an expert on them than you. 

I think MARTY BALIN wrote that one..., and assume it's him singing for that reason and it sounds like him, but I often think MARTY and GRACE are interchangeable...   

I think THE TEMPTATIONS not only had many lineup changes, but multiple lead singers over the years, and at times, I think, all together: DAVID RUFFIN, EDDIE KENDRICKS, OTIS WILLIAMS...
http://www.youtube.com/watch?v=rQdSJXPDtjs 
http://www.youtube.com/watch?v=leD9kTMWS9M&feature=related

[Mr. DS]

Hip Hop has some good examples

En Vogue
http://www.youtube.com/watch?v=8pwOiC8aJrs

NWA NSFW (link disabled)
http://www.youtube.com/watch?v=TMZi25Pq3T8