Sometimes the USB enclosure alters the addressing, causing the partition not to start at the proper spot.
Any way you can hook them up direct to SATA?
Wouldn't hurt to collect diagnostics with them attached and attach the zip to your next post.
In this case, either drive should be mountable, because as long as you used the parity1 slot, the two drives should essentially be a RAID1 mirror.
It's possible only one drive will SHOW as mountable, because the XFS signature would be identical keeping the second drive from mounting.
Since you only had 2 drives, you actually might be ok.