try a variety of ways ...try seperate and togather or bi ....some individuals have good ear and can match the note of others ...others they just can't do it if thier life depended on it...I suggest try both ways ...if they have experience then let them do it seperately but remember when makeing them listen back to each others tracks and haveing them singing on top of each other...make sure you have what is playing in the headphones loud enough that the singer who is sing does not drown out what he or she hears...turn down thier mic and have them harmonize...plus harmony is not achieved in a dead room so ad a little reverb to the singers voice who is being played back and a little less to the singer who is doing the recording at the time....the ears hear harmony from reverberation...thats why a chrch or hall is so harmonic in nature......ok later got to go hope that helps theres a dust lint flying too close to my mic got to put up the shield....I'm still paying for the gear you know

Novice here.
I think I would send harmony 1 & 2
to one side, 3 & 4 to other side of
their headphone mix.
not hard panned just perhaps 1/3?
If one or the other's monitor mix is
louder than the other, I would think
someone is going to over compensate, either by over singing, or filling up the gaps in nuances, he or she can't hear from the others.

I have put choral rehearsal tapes [cd's] together by getting all parts on separate tracks, and then making cd's with featured part on one side and the remainder on the other side. This gives the user the option of controlling how much her/his part stands out by using balance adjuster.

Bruce P.