Double disk failure is improbable but not impossible.
The most impressive thing is that there seems to be no dataloss, almost whatsoever. Whatever the backup system is, it seems rock solid.
It's not even improbable if the disks are the same kind purchased at the same time.
Not doing it for this reason but rather financial ones :) But as I have a totally mixed bunch of sizes I have no RAID and a disk loss would be horrible.