The CK61 uses MIDI CC [18-26], [77-86] and [111-119] for its organ drawbars, for section A/B/C respectively.
The YC88 uses [102-110] and [111-119] for upper and lower, respectively.

So there's some overlap here on [111-119], meaning I can use the CK61 drawbars to control the YC88 organ, provided I'm using Section C on the CK, and set the YC to play on the lower manual. All good when you're using a single organ manual.

One thing that's a bummer with my setup is that when you use 2 manuals (YC88 lower, CK61 upper), the only way to get CK drawbars to control the YC's footage is by using CC [111-119], which corresponds to the Lower drawbars of the YC, not the Upper. It would be so great for each keyboard to use its own drawbars to control footage for the appropriate YC manual.

(I suppose people who set up the other way around - a CK88 lower and and a YC61 upper - would be in luck here)

I get the sense that what I'm asking the CK to do is above its pay grade; I've hunted around to no avail, but posting here in some vain hope that I've overlooked something, and there's a simple solution to this, without needing some intermediary

The Reface YC can do exactly this because its drawbars are [111-119], so makes a great upper organ manual for the YC - apart from being too small.