Alex, I feel your pain. And am feeling a teensy bit smug about it. Sorry. :-)
As of about fifteen minutes ago, my theremin has a volume antenna and a volume pedal (see page 27 (http://www.thereminworld.com/forum.asp?cmd=p&T=1420&F=557&p=27) of Gordon's Progress for reason), a pitch antenna and a relative pitch pedal for the second voice, provided by the PS-5. I preset the interval of the second voice on the pitch shifter - either a semitone, a tone, a perfect fourth, a perfect fifth or one or two octaves above or below the primary voice. With the pedal rocked to the off position the two voices are in unison and sound like a single voice, but are available separately for subsequent processing. With the pedal rocked full on the two voices are separated by the preset interval. With the pedal part way rocked the voices are separated by part of the interval.
Rocking the pedal back and forth during a long stroke is neat.
Of course there are limitations. One could add more voices with more pitch shifters and expression pedals but that would be an expensive way of going about things and one would still be limited in the choice of intervals.
But it is one step closer to a polyphonic theremin. I know the set of intervals I would like to try out first. For that see the long posting on page 20 (http://www.thereminworld.com/forum.asp?cmd=p&T=1420&F=557&p=20) of Gordon's Progress, but ignore the last paragraph and replace with "Part three of the diagram illustrates a suitable arrangement for a pedalboard.")