Not as unlikely as many people would like to think! If the two drives are from the same production lot, they may suffer from a common manufacturing defect. And because they are in the same chassis, if a server fan fails, both drives may subsequently fail due to thermal damage within a very short interval.

Even if they don't fail simultaneously, the mirror drive may fail (or even more likely) have read errors or flipped bits that will corrupt the restore or render it impossible.

Personally, I don't place much trust in any RAID configuration other than RAIDZ2 (ZFS; you can lose two drives and still recover all your data; every block is checksummed to avoid reading or restoring corrupted data).

But even ZFS can't protect you against accidental deletion, fire, theft, or earthquake.

I do RAID data recovery for a living. Customers always think that '2 drives failed at the same time'. Thats not what usually happens. What really happens is that 1 drive fails and the raid does what it supposed to do - it keeps on functioning. Sometimes for months. Then the second drive fails and it stops. They call in the IT guys and it appears that '2 drives failed at the same time'. Not.. Most raid data loss is caused by IT guys trying to fix it - because its a raid and its redundant and it cant be hurt so let me start swapping stuff and rebuilding and destroying...