They get the same writes but not the same reads, so depending on the bug source it may not hit both at the same time. The read pattern itself may affect the way the writes are performed to the flash (delaying or speeding up writes pending for commit) that it may have a butterfly effect on the rest of the behavior and removing the disks from being in sync with regard to firmware bugs.

If you'd still follow up on your idea of using a read-only root like you did with CF cards and figured a safe place for the logs you could still use the SSDs in the same mode. Why not go that route?