In RB you can set the Harmonizer to follow different MIDI tracks so you can 'play' the notes you want the harmonies to 'sing' on a track beforehand, then tell Harmonizer to use those tracks for each voice.

I still like to do them one at a time so I can tinker with the other settings available, like male/female and portamento for each 'backup vocalist' part.

I do not work here, but the benefits are still awesome
Make your sound your own!