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.
A long time ago we had a Dell server which was pre setup raid from Dell (don't ask, I didn't order it). Eventually one disk on this server died, what sucked was that the second disk in the raid array also failed only a few minutes later. We had to restore from backup which sucked but to our surprise when we opened the Dell server the two disks had sequential serial numbers. They came from the same batch at the same time. Not a good thing to do when you sell people pre configured raid systems at a mark up...