For years people have been dreaming about, and discussing, the polyphonic theremin. FORGET IT! Just look at the difficulty we have playing the single-voice instrument. Can you imagine attempting to control volume and THREE voices? You would have to be the multi-armed Hindu god Shiva to do it with anything coming even close to precision.
If you want a polyphonic theremin you can plug your Etherwave into a harmonizer or pitch shifter of some sort, but what you will end up with will be more of a polyphonic effect than a workable, practical, musical instrument.
A number of years ago, an inventor came up with a two-voice theremin which I personally experimented with. It was basically a conventional theremin with an added third dimension.
In order to get around the problem of interference between THREE independent electromagnetic fields (one for volume control and two for pitch), the inventor came up with the ingenious idea of using echo location of lateral pitch arm movement for the second voice. Voice one was controlled conventionally toward and away from the rod, but the second voice was controlled by right/left movement of the same arm (volume was controlled via the loop, as it is with conventional theremins).
On second thought........if I remember correctly.......it was in fact the VOLUME ARM that controlled the second voice (by lateral echo location) as well as volume (by proximity to the loop). It must have been that because lateral arm movement of the pitch arm would have interfered with distance from the rod, which would have made the two voices impossible to control simultaneously.
It was a great idea and fun to experiment with but it wasn't feasible. Charles Richard Lester, who tried out the same prototype instrument, came to the same conclusion as I did.
As Thomas Edison said, "I have not failed. I've just found 10,000 ways that don't work".