WELL, 
i've got a tune up in 'Songs' forum, that has my own double tracking of lead vox going on.....
not that this is my 'best' example, but it is an example.
 

Do what I do- DON'T do it in one take xD
Actually for me, I tend to try to records parts that are the same energy at the same time, so my recording sessions are a bit 'jump around'

Thanks Bats and Dav for your excellent thoughts!

Multiple vocals? You mean like this?
 
http://www.reverbnation.com/rockinrobby/song/22423313-without-you-12-2014
 
It starts out as one, but goes to "many..." I can explain in detail how I did it.

Philip, I think this can be done very tastefully.  Lennon and McCartney were masters at trading off on Lead vocals.  McCartney taking sections of a song where Lennon couldn't hit some of the higher notes, and vice versa.  A Day In The Life, excellent use of blending two different songs into one. 
 
I think the thing that holds it together for me is each vocalist having the same consistent part, so that you anticipate who is singing next.

Thanks Jab and Makeshift for your excellent thoughts! Jab, I'll try to listen later.

Thanks Jab and Makeshift for your excellent thoughts! Jab, I'll try to listen later.