Just a quick update for anyone who has a Late 2012 Mac Mini server i7 processor with dual internal HD.
I think I’ve tried every permutation to get the system to boot with the two drives in a raid configuration including converting the volumes to APFS. Absolutely no love. Mojave install disk will let you install the OS to the OS to the APFS raid drive, but the drive goes into a never ending reboot cycle.
End of the day. I have still not found a way to use the two drives in the Mac Mini server as a Raid array since Sierra.