There are some songs that have been remixed by the author, so you can assume that at least some parts of the vocals in both songs are the same. What is the order of operations using a program like Audacity extract the vocal track.
It is similar to this question https://video.stackexchange.com/questions/1947/subtracting-specific-audio-using-an-isolated-track-from-the-same-mix but if you do the subtraction it will result in combining the instrumental part of both songs (I think...) which is not what I want.