@deckingman Thanks for the insights! You're referring to some quirks I hadn't seen in the hardware overview and wiring diagram pages, but I was able to find more info here on the forums. I certainly appreciate that the docs mention several of the limitations in place today may well go away with future firmware revs.
This exercise has been a good case study in reading the fine print and balancing conflicting features. I was able to create a spreadsheet "simulation" of a few different board combinations based on the port specs and known limitations. Fortunately I still have a path to success without giving up anything I really wanted. After putting it all together, one gets a really interesting view of the balancing act the hardware devs at Duet3D take on when creating new products.
Reminds me of my college days when I had to choose between HW and SW eng. I ended up choosing software, in part because you can almost always just release a bug fix to handle your dumb mistakes (and also because the EE classes started at 6 in the bloody morning!)