I played with this patch about a week ago and it worked relatively well for a cheap-ass stab at a poly set up. An important thing to mention is that the note lengths at the MIDI input should be as short as possible, or a last-note-priority MIDI->CV converter should be used, especially for simultaneous notes/chords/whatever. I had Ableton Live's MIDI plugins working their magic trimming the note lengths down. Another downside to this setup, therefore, is the lack of control over the played envelope, but you can't have it all!
The above is setup for "duo-phony" but could easily be adapted to four voices (if I had more mults....)