Well, there is something, not very elegant, that may work. I did it once in a digital audio workstation class with decent results. Basically it works with your average pop songs without a bunch of reverb or backup vocals. Because most pop songs pan vocals to the center of a stereo mix, you can use a technique called "vocal removal". Most audio editing programs have the ability to do this, but don't expect high quality results. Basically you would be removing what is common to both tracks (i.e. the vocals), leaving behind what is different (i.e. the instrumentals).
Basically you split the stereo track into its left and right channels, make both mono, invert all (or a selected part) of one of them, and play back the result. Sometimes it works well, most of the time it doesn't because lots of other things could be panned to the center, and they will go bye-bye as well.
I found this
plug-in that might work with some Mac programs.