RAID-6: better to replace two dead drives at the same time, or one at a time?

Solution 1:

!!!!! ONE !!!!!

Do one at a time, seriously dude, don't think of doing this ANY other way ok.

Anything else will test your full system restoration skills.

Solution 2:

Do you have good, recent backups? If not do you think you can get them in reasonable time?

I'd honestly be more concerned about tripping the bad drive offline during a rebuild than anything else - If you're already throwing SMART errors you're more than halfway there.

My suggestion would be to confirm your backups, then rebuild one drive at a time to try to recover to a state where you can replace the one throwing SMART errors (dead drives first, soft-errors last).

If you have no backups it's a crap shoot: Backing up may create enough soft errors to mark the marginal drive as failed, as may trying to do a rebuild.