I just purchased a dual bay NAS and I also purchased two identical Western Digital 2TB hard Disks (model WD20EARS to be exact). They are both mirrored using RAID level 1. Now I've been told by a friend of mine that I should have two different brand drives of the same capacity instead of identical ones just in case they are both faulty and both die on me at the same time. Obviously I'm a bit paranoid that he's right but I want your opinions please.
There is probably a very slightly elevated chance of both drives failing AT SOME POINT, but prematurely, if they are from the same batch and it was a bad batch. I can't see 2 drives failing at the exact same time. If hard drives lasted exactly 1000 hours or something, sure.
As long as you have another copy of your data somewhere else, I wouldn't worry about it. Or if 1 drive fails, get another & rebuild ASAP.