How to double voice effect like this singer?

So for the past couple of weeks i got into this band called "Cradle of Filth" and the singers voice like during most of his songs are just breathtaking, and one of my favorite is his "Double voice" so how do i make that noise in audacity? Heres a link to a song that is a perfect example of what im talking about :D http://www.youtube.com/watch?v=qRNfeMaUBbo or http://www.youtube.com/watch?v=PUvJoH5YSH8 all answers are awesome!