Why are there RAID models where there are more than one parity disk?
Two parity blocks are needed for RAID-6; one is a standard XOR over the data blocks as in RAID-5, while the second involves math that's over my head.
The second type of parity allows for a recovery in a case where two data blocks, or a data block plus the standard parity block, are lost.
If you're referring to RAID 6 then it has two disks for parity for redundancy. It allows for 2 disks to fail and the array to survive, as opposed to one.