This may sound amateurish and unprofessional these days, but I love when back in the 60's one vocal was recorded on one channel and other vocals were recorded on the other channel. I think that independence of the vocals gave the songs a fresh sound, as if it were a live experience. These are some examples that come to my mind (please listen to the songs with headphones):
"Rain" (The Beatles, 1966): John Lennon (lead vocal) on left channel and Paul McCartney & George Harrison (backing vocals) on right channel.
"For What It's Worth" (Buffalo Springfield, 1967): Stephen Stills (lead vocal) on left channel and Richie Furay & Dewey Martin (backing vocals) on right channel.
"Christine's Tune" (The Flying Burrito Brothers, 1969): Gram Parsons and Chris Hillman share the lead vocal, the former on left channel and the latter on right channel.