The only workaround I came up with that works was to install qubesOS and its /boot/efi and /boot on the SSD, and to install KDE neon and its /boot/efi and /boot on the SATA drive, and make both of them bootable. Then I have to choose which drive to boot from in the BIOS menu. That way everything works fine.
... which leads me to my next point.
Congrats to the developers on this new version. Now, I have to admit that when I tried out Qubes a few months ago, it was on a box with a traditional SATA drive, and I know that affects response time, but the difference to me on this new box is night and day. The responsiveness of Qubes the first time I set it up was so bad it simply was not viable. This new setup is pretty snappy.
I'm sure I'm going to have all sorts of questions as I tweak this for my own use, but I gotta say right off the bat that this is a real, usable system this time around, and not the sandbox "well, this is an interesting idea" kind of thing it seemed a few months ago.
Thanks to the developers for an interesting OS that combines security *and* usefulness.