1 korg monotron + 1 korg monotron = 1 duotron mod
I made a small experiment.
I soldered two wires on the test points labeled GND and GATE, on each monotron. If I connect both monotrons, GND to GND and GATE to GATE, one monotron can trigger the other by touching the ribbon. By tunning the Pitch knobs, I put one monotron one octave below, like having a sub oscillator. I took the wires out near the pot for ribbon scale, here is no need to drill holes in the plastic case.
If the audio out of one monotron goes to the audio in of the other, there seems to be enough to connect only the GATE wires. Both monotrons should have batteries of the same capacity, otherwise the monotron with discharged batteries may sound continously on lower pitch, when the ribbon is not touched.
I didn’t succeed (yet) to tune the ribbon scale pots exactly, so the pitch ratio drifts at higher note… but this is the "beauty" of analog synths. 🙂 The pulsating sound happens because the 2nd monotron has audio in connection, thus it will trigger the LFO continuously. If I would connect both to a mixer, it would not pulsate. You can see the test points GND and GATE bu searching on the web "korg monotron pcb".
You can try this at your own risk, I’m not responsible for any damage. Watch out to not short together any test points. For e.g. the GATE testpoint is very close to a small testpoint, it’s very easy to solder them together by mistake.