Frequency division multiplexing:
Each circuit has an oscillator (relatively low frequency) connected to one electrode, and a tuned amplifier connected to the other electrode, and a synchronous demodulator following that receiver, the output signal being proportional to the resistance between the electrodes. You would then use that output to drive your MIDI transducer. The frequency selectivity would cause it to reject signals from another electrode, which would be at another frequency.
Time division multiplexing:
Generate N pulse trains with duty cycle=1/N, where N is the number of transducer pairs, and each pulse train would be at a different phase (time slot). You could then synchronously demodulate the pulse trains (the demod acts as a sample-and-hold), and feed each output to its respsctive MIDI transducer.