RAID 1 Hardware show two disks

((I've never used OMV-Extra's HWRAID repo but, after activating it, I didn't see any additional plugin's. Perhaps someone else, in the forum, has information on this Repo.))
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If you configured hardware RAID, at the BIOS level, OMV shouldn't be able to see two disks. The entire idea behind a hardware RAID controller is to remove disk hardware details from the OS and handle them with hardware and the controllers micro-code. In the bottom line, a hardware RAID controller presents the ARRAY to the OS as "a single disk". Outside of a software utility that can query the controller, the OS shouldn't be able to see individual disks. Are you sure it's configured correctly?

What you're calling a "driver" would be a utility for a direct access to the hardware RAID controller. The availability of such a utility would depend on whether HP created one for Linux, or not. In this case, if an open source utility is available for your controller, it would be handled at the Debian, Jessy level. You might find something here - > Debian HP RAID
Otherwise, to the best of my knowledge, OMV's programmers / maintainers do not create and / or support brand specific hardware drivers or utilities.

As a suggestion, you might want to consider using OMV's software RAID. If your RAID1 array is created in OMV, you'll be able to see the individual disks, SMART info, the health of the array, repair processes, etc.
If you go this route, and in the off chance that your HP Gen 8 platform goes belly up, it's possible that your RAID array could be transferred to another PC with a new OMV build. On the other hand, if you commit to HP's hardware RAID your disks are married to that particular model (or family) of HP RAID controller.