There are some neat tricks you can do with the 2 stereo signals. Linear combinatios of L/R channels is the easiest, but doesn't work well with modern mixes where bass/snare/voice is centered and other instruments are rarely ever hard-panned (you can get really cool results on some older albums like Rubber Soul - there's a lot of hard panning going on there)

There are of cuorse more advanced techniques for isolation of centered tracks and vocals in particular see http://virtualdub.org/blog/pivot/entry.php?id=102 and http://dsp.stackexchange.com/questions/1499/how-to-extract-v...