Got the Pi 4 set up with the two drives on a hub. Currently configured as raid 1 using OMV. Not especially difficult to set up although I have discovered that the Pi 4 doesn't boot or reboot reliably with the USB hub powered. It works fine as a passive hub, but I suspect it may struggle if there's a lot of IO and definitely if I want to add more drives.
The most likely theory is that the Pi 4 doesn't like power coming back down its rails from the hub, so I may see if I can snip the power wire in the cable*.
Not convinced by OMV for my system. It's a very pretty piece of software with loads of features but probably overkill for me. I'm also not at all certain of the benefit of raid 1 when I only have two 1TB drives. I'll probably blow the whole lot away and just set up some Samba shares.
*which I've since discovered
may stop it connecting.
EDITED: 28 Apr 2020 17:50 by WILLIAMA