First: Each speaker has a separate channel. A part of the song was on the left channel and another part was on the right channel. I've only heard this used in songs to create a temporary effect, not the whole song.
Second: What the f
uck is this shit?
Edit: You can do this in Audacity (if you don't know what it is Google it yourself). Click the highlighted selection.
http://img651.imageshack.us/img651/6081/splite.pngCaveat: There is no way to do what you want if both the left and right channels aren't unique. It's like trying to make a .psd from a .jpeg. This isn't an effect created in post from a compiled song. The artist sings separate from the instrumentals and they are combined afterwards with the artist in one channel and the instrumentals in another.
Edit2: 'Left and right channels' correspond to left and right speakers.