It just crossed my mind, what if there was a theremin with the two antennas on the same side? You could play with only one hand: one axis for pitch, one axis for volume. Up and close to the instrument for a high-volume high-pitched sound, down and far for a low-volume low-pitched sound.
And you could make a theremin with two of these on each end: a stereomin. A two-handed player would be able to do two notes at the same time, and therefore chords, or accompaniment and main melody, etc.