Duophony ?

[k brown]

It would depend on the synth - some were high-note priority, same were low-note.

[trogluddite]

I'm almost afraid to ask what happens when the low/high order changes on a held note

That would depend on the implementation. When the modulators (i.e. including envelopes) are shared, you'd have to choose either retriggering for every order change (i.e. possibly at note-offs as well as at note-ons), or keep the envelopes sustaining until all notes are released. As it's technically simpler to do, I suspect that it would be the latter in most cases; so that you hear the oscillators playing musical chairs.

[wlangfor@uoguelph.ca]

I'm kind of surprised that apparently nobody's gone down this road yet - duophony results in some interesting playing styles that a polysynth can't do. But of course most synth players these days let the sequencer do the 'playing'.

I agree, they sound nice. I believe that absynth and dune have done this.

[k brown]

https://www.soundonsound.com/techniques/duophony