The problem is that both devices attached to the JMicron based controller have the devicefile /dev/disk/by-id/usb-JMicron_Generic_0123456789ABCDEF-0:0 which is falsy, because a devicefile MUST be unique. Because OMV uses the /dev/disk/by-id devicefiles by default (because this is the only predictable one, /dev/sdX can change depending on timing issues during bootup, ...) the disks page only shows one.
OMV has already included UDEV rules to workaround this, but it seems not to work. Sadly i do not have such a crappy USB controller, so i can not test it. Because of that someone in the community must get it working and create a pull request on Github including the fixes or open an issue describing exactly what to fix.
You can find the UDEV rules here at /etc/udev/rules.d/99-openmediavault-dev-disk-by-id.rules or https://github.com/openmediava…ault-dev-disk-by-id.rules
If the rules did not work it might be because of a missing full path to the serial_id command. It is located in /usr/lib/udev/serial_id.