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.
each server has a pair of mirrored disks, so it seems we're talking about 4 drives failing, not just 2.
On the other hand the primary seems to have gone down 6 hours before the backup server did, so the failures weren't quite simultaneous.
Yes—I'm a bit unclear on what happened there, but that does seem to be the case.