If it's the old Chuck Britz stereo mix of Don't Worry Baby, what you probably did was turn the balance knob. The lead vocals were panned one side, the backing vox the other.
Regarding cutting out voices using eq, impossible. You can highlight certain frequencies, but in the case of bass vocals, turning up the bass eq will make everything sound muddy. You can Oops some stereo recordings
http://en.wikipedia.org/wiki/Out_Of_Phase_Stereo, and this
may isolate some vocals, depends how they were mixed.
Your best bet for what you are asking though is a program called Melodyne. This breaks up a recording into seperate frequencies. You can then delete parts as you wish. It doesn't sound perfect though, but as a research tool it can be pretty cool.
Hope this helps. I'm sure more technically minded folks can add to this