Hi I do that as well, but usually when I double up I add differing amounts of reverb and echo to each track and sometimes Eq, removing the high frequencies from the left and removing the low frequencies from the right and boosting the harmonic range (the very top).

I used three different vocal tracks with Joice, seperate takes and with a bit of twiddling provided some very nice harmonious singing ... there are no real rules but knowing lots of 'tricks' can be useful and add a different dimension to the finished song.

http://soundcloud.com/the-old-grey-wolf/joice-marie-and-the-old-grey-wolf-ltd-co-keep-movin-on

http://soundcloud.com/the-old-grey-wolf/joice-marie-the-old-grey-wolf-ltd-co-from-a-lush-garden