On v3.4, I was able to get everything working without issue, from your suggestions. Thanks!
It was all pretty straightforward - define a CoreXYUV, map the axes and endstops, and verify it all as a CoreXYUV, first. Then set up to the tools, where the second one maps (U, V) to (X, Y), and ensure they work as expected with T0 and T1, where each of these parks the toolhead.
Seems all good... with no pain, surprisingly, so far. I know the XY offset calibration to come will be the bigger pain.
I don't have the modern Duet boards to make use of this code, but it's still exciting to hear that this is coming along.
Today I released a complete open-source Dual-Gantry CoreXY printer called Dueling Zero, w/configs, STLs, videos, CAD, docs, diagrams, the whole nine yards. The GitHub repo includes a sample RRF config for XY motion, which "just worked"... it was nice.
https://github.com/zruncho3d/DuelingZero/
Curious what you (and others on this thread) think. In particular, the section on collision detection and avoidance in a shared workspace is general for any firmware.
https://github.com/zruncho3d/DuelingZero/blob/main/SOFTWARE.md
I'll share it all on GitHub once I've addressed a few minor items, just like all my other printer mods:
As a teaser, imagine two BoxZero gantries, with one rotated 180 degrees, plus all-custom XY joints so that both gantries can have shared rails.