That's the best choir patch I've heard from the Rev2. It sounds like a combination of two patches. How is it that they merge and separate at different times? Modulation wheel? Keyboard pressure?
It's actually just a single layer... and as always with synths that have no way to control formants, the bass notes get more and more dull, sounding more like a kind of ensemble... but that seems to work well as a blend in... maybe that is what you can hear as "crossfade" (I think).
If it's the loud parts that are more staccato, then it's simply a matter of modulation via velocity... the more velocity, the more I open the filter, and also let in some sub oscillator to fatten it up (sounding a bit like if the males in the choir raise their voices)... also more vibrato is introduced as velocity goes up.
But the trick that gives this sort of "formant timbre" is in fact the build in reverb... it's actually 100% wet to be able to achieve this, and this is also why the sound is a bit "smeared" and "dark"... and also why the choir seems a bit more set in the background...
It's not perfect... and it never will be on an analog synthesizer unless SCI introduce a formant filter of some kind... on a per voice basis... but with other sounds playing at the same time, the imperfections is easier to hide, and the way it sounds also lets it sit well in the background not being too obtrusive... so the limitations can also have their advantage
hope that clarifies a bit... creating this patch required serious minute tweaks to A LOT of parameters to sound "just right"... it involves both resonance, audio mod, reverb and a lot of other things including parameters that require change according to key position...