Guide 1 there should also get you going. In your case you would have to run it on disk a and b. It will not touch your data partitions and if they are ok the span should rebuild in 10 minutes or so.
I would try gdisk first and if that doesn’t work, try guide 1 from the link above on both disk to write a clean OS to both disks. After you install the OS on the first disk, shut everything down and hook up the second disk and follow the same process. Pop both drives back in the enclosure and cross your fingers.
