From what I've run across, most of the time when a cdg manufacturer advertises CDG Multiplex, it's simply having the demo vocal on the same track as the background instead of duplicating the song with and without lead vocals. You then switch the vocal on or off, or depending on some players, turn the right channel down to get rid of the vocal. May not hold true 100%, but those I've seen are that way.
This has confused some people where Vogone is concerned because they truly believe that their $99.00 home player will completely remove vocals with the turn of a switch, and Vogone can't
George