You're not using FreeNAS properly if you're using hardware RAID mirrors. It uses the ZFS file system which is software RAID and you lose some of the benefits if you present it with logical disks from hardware RAID.
If you can create logical disks for each of the physical disks and present them to FreeNAS individually that would be OK.
If you can create logical disks for each of the physical disks and present them to FreeNAS individually that would be OK.