I have 2 idea's!
This would be a special split-mixer option (I got this idea from the UDO Super 6, another great synthesizer of this era):
This could be done per timbre and multitimbral.
Single timbre:
Have the 3 oscillators split on the keybed with a special "mixer" setting. You would go to the "more" button on the mixer and put "Oscillator keyboard split" on. This would add more options in the mixer menu.
X - Y - SPLIT POINT - CURVE
(you scroll on X which audio source you choose= OSC1,OSC2,OSC3,RING MOD, NOISE) and Y the same.
You could add a last option to choose what kind of curve if that is possible (EXP/LOG)
You can't select same Y as X.
On split point you select the note it X-fades
On Curve you choose how the curve fades in & out. (from 1 seminote => 6 octaves)
When you finish this a new X SPLIT Y menu gets added. In this one you can't choose the previous X&Y and the SPLIT POINT value can only be higher then the SPLIT POINT.
Multi timbre:
This we select from the VCA menu, we scroll down under the SYNTH1 SPREAD.
We have X-FADE TIMBRE (ON/OFF)
When we turn this ON:
A new set of settings get added in the VCA MENU under spread:
X - Y - SPLIT POINT - CURVE
X & Y can be SYNTH1, SYNTH2 & SYNTH3
the rest is the same as single timbre.
This is at cost of voice usage! (As you would play double the notes on the split points, but they play at a certain level)
So it is to the player to limit his voice count on the different timbres properly.
SPLIT PAN:
This is an option added in Polyphony.
SPLIT PAN (ON OFF)
When unison is turned on (Only possible to go from 2-4-6-8-10-12-14-16)
It would play max 8 voices poly(when you choose 2) but playing 1 voice hard pan left, and 1 voice hard pan right.
This is something that Stereo Futures pack did, but he has 2 duplicate timbres. 1 hard pan left 1 hard pan right.
With this option you could have the same on 1 timbre and have 2 timbres left to add more magic!
This split pan creates a slightly wider sound, as every voice sings slightly unique!