I'll give you a view:
Poly chaining or polyphonic mono synth chaining in my opinion has 2 (minimally), or upwards of 4 or 5 (ideally) touch points (numbered below).
Some of the early, somewhat primitive Poly synths had modes where different timbres per voice could be played to beautiful and very much controllable effect. (watch Dave Spiers Oberheim 8 voice video:
https://www.youtube.com/watch?v=dNQfzF2LvSs ). But that's not what most are after.
What most of us need is #1, #2, and part of #3 but we want it all...
1. Common synth architecture
- # of OSCs per voice, waveforms, envelopes and modulation features
2. Polyphonic note dispatch/distribution
- note stealing/note priority improves the experience but is more difficult to pull off. Many of the early Polysynths (some of the Roland Juno synths) simply allocate voices then refuse to play the n'th note after all of the voices were allocated. The Kurzweil 250 was the king of note stealing since it had an advanced algorithm which would steal a chorused note before it stole a note which was consider significant to the overall chordal structure. But then again, it cost > $10,000 so you would expect this type of rocket science in 1981.
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3. Common performance control scaling
a. Pitch bend (requires some bit of calibration or at least, common # of steps on each synth) when you bend pitch a whole step, you expect ALL notes to bend by the same margin else detuning will result in horribly unpredictable ways
b. Modulation based on a common LFO (and more importantly, a single running waveform) Most mono synths have their own onboard LFO and if used for plus width modulation you may slide by; if used for pitch vibrato, mayhem will result unless all of the OSCs are modulate by the same master LFO.
c. Pulse width modulation (as above)
d. Good, but not great portamento/glide (check out MC's feature on this ala OBX). The best polyphonic glides were analog and imperfect such that the circuit would arrive at the target note in a way that was not quite 100% at the same time.
4. Realtime parameter editing - the Slim and Little Phatty combo was the king of this, move a pot and get lovely lights dancing in unison.
5. High number of voices - envelopes with mid to long release really demand more voices than are played; last thing you want is a voice to cut out abruptly.
To cut to the chase, the LP / SlimPhatty poly chaining does #1 perfectly, #2 decently, 3:'a' only, 4 perfectly, and 5... well, how much $$ do you have? If you know Pete Brown, you may have stumbled across his 6 voice Slim Phatty demo. It's very very nice and sounds as if it came right from Beck's recent award winning Morning Phase album:
https://soundcloud.com/psychlist1972/mo ... -6-voice-2
Anything which ties a Little Phatty to a SubPhatty or Sub37 is going to have gaps as the newer synths have features which the older do not such as subosc and more complex envelopes; the filter response/overdrive sonic quality may also differ. And unless tons of effort is invested in all platforms, 3b, 3c, 3d (above) is going to be difficult if not impossible without external CV and heaps of software enhancement. #4 is likely doable assuming the CC #'s align (which they ought to) for synth features that are in common between synths, and #4 always applies.
Werkstatt; maybe there is something there but it's raw and needs a good poly Midi -> CV interface. It's primitive enough that maybe you could put something together a low cost but it's still going to cost you a few thousand for an 8 voice and I'm not sure there is adequate temperature compensation to support multiple voices sounding well across a big enough octave range. No experience here so others will need to chime in. I guess that's why Dave Smith sells so much gear. The Prophet 08 was something special even if it was based on IC based filters and Oscs.
Oberheim threw a nice nugget into the Matrix 1000 back in the day; at one point, I had an Oberheim 18 voice. yes, I know, I know, it was DCOs but still, I had a Matrix 6 keyboard with a Matrix 6R and a Matrix 1000 all loaded with the same patch set. If you know the Matrix 6 family, you'll know that this was a very uneasy and awkward time for synths because nearly all of the manufacturers had moved to a parameter map interface / numeric keypad... but the Matrix combo yielded a very very nice Analog poly that worked with no fuss.
What's next? I don't know but I can tell you from experience that this is much more complex than meets the eye/ear. Your minds eye wants this and sees it as easily attainable, your ear will turn sour the second a decaying not has it's legs cut from under it, or when a pitch bend results in a disaster that sounds like a 4th grade band playing a Hanukkah song for the first concert.