Recently I acquired a Warm Audio Ringerbringer, a recreation in a smaller housing of the Moogerfooger 102 Ring Modulator pedal. Interesting things happen when one theremin is plugged in as the carrier signal and another one in the audio input and the two theremin sounds modulate and interfere with each other. The ring modulator heterodynes the two audio signals and produces sum and difference frequencies, similar to how within the theremin the RF signals of the variable and reference oscillator are mixed and heterodyne to produce the difference frequency within the audible frequency range. The sum frequency is RF and gets filtered out in case of the theremin.
Short outtake version on YT (44 seconds)
Full registration (9:11 minutes):
https://youtube.com/live/geN3Mw-s4is