a human voice/audio sound instrument as a monophonic instrument modulates/triggers the audio/midi of the guitar input as an expression controller somewhat similar to a breath controller, but with the volume and frequency of the input tone
a guitar itself plays a chord or triggers a definable chord with a played note (e.g. within a scale) and thus modulates the second audio voice input like a choir singing
for use case 2:
A vocoder application would certainly be possible, but I was thinking of something else: harmonising voices with guitar chords, such as here as a hardware solution from TC electronic: https://www.youtube.com/watch?v=bSoC2ivmmv8