Last night I finally plugged everything in together for the first time. I had lots of problems getting the Pi to boot properly and produce an HDMI signal so I could actually see what was happening. Eventually I got it sorted, and was able to configure OpenMediaVault. Halfway through trying to set up the two disks as RAID1 I seemed to have more problems, and then stumbled across lots of stuff about RAID1 being effectively useless when used with USB disks (presumably due to the need to share the bus, rather than having independent controllers like SATA - IDK, I'm not an expert). I also thought a bit more about what I actually want the NAS to achieve, and maybe RAID isn't it.
So tonight I'll start again. This time I'm going to ignore RAID altogether, and set up a Main disk and a Backup disk, and use OMV to schedule automatic, incremental backups of one onto the other. So I kind of get the dual redundancy I wanted, as well as some protection against human error like accidentally deleting files.
Oh, ps. the fail-over power didn't quite work. Upon pulling the plug, the Pi instantly rebooted. But at least that means the batteries had taken over and were able to supply the power required. I guess the Pi's SoC has a brown-out detect that got triggered by some minute glitch. Something to fix later.