Try changing the USB connections for the drives. Also alternate the first drive. As the HUB works (No idea why) try in different USB ports USB2 and USB3.
I plugged both drives into the USB2 ports and it mounts both fine. It also seems as if it's the same USB3 port that fails when the're both plugged into USB3.
What kernel is Debian using? .. we're on mainline 5.4/5.5 now and I suspect they're still using the RK 4.4 kernel.
Sorry I don't know how to find the kernal info but this is the install wiki Rockpi4/Debian - Radxa Wiki