For isolating what is common to two tracks you could try …
Robert J. H. 's stereo tool plug-in … https://forum.audacityteam.org/t/karaoke-rotation-panning-more/30112/1
or Kn0ck0ut plugin [ windows only ] … http://www.freewebs.com/st3pan0va/
Don’t get your hopes up though : isolating the bits you want without any of the bits you don’t is rarely possible.