The ReadyNAS should offer RAID 6 as an option, and that does allow for two simultaneous drive failures. Any two drives could fail at any time and data would still remain. The difference though is the ReadyNAS won't downgrade the RAID level to 5 or 10, shrinking the array. It will keep data in a RAID 6 stripe, and will expect at least one drive to be replaced to get back to a level of redundancy. A Drobo Pro left alone with 2 drives dead would eventually return to redundancy, as long as there is free space.

The Drobo units really like to mix and match RAID levels, and migrate between them. This works ok in higher end enterprise SANs to still keep performance, but in a small box like the Drobo, it's mostly helpful to facilitate the ability to mix various sized drives, at the cost of performance.