As my sig indicates I use dual parity on all my servers that have 12 or more data disks, although 2 disks failing at the same time is rare, it happened to me on 2 occasions during a rebuild, not a complete disk failure, but a few read errors on a second disk during the rebuild making some of the rebuild files corrupt, at the time dual parity was not yet available.
Also feel that checksums are invaluable, not because of bit rot per se, but for when a problem like above happens, or a disk redballing during a write operation, without them there's no way of knowing if or which files are corrupt.