The backups are *not* spread across more than one disk. Doug has his multiple disks being backed up onto multiple backup disks but is set so it's a 1:1 source:destination.

You cannot have a Crashplan spanning disks natively. It would have to be a RAID or JBOD combining more than 1 disk in to one larger virtual disk for that to occur.

The bolded underlined statement about redundancy is not correct. It can be no less redundant than any previous backup system. Taking backups of backups is not the best backup solution IMO. Keep multiple stages of backups instead.

If you want redundancy in your backups then add multiple destination disks to each backup set. If that's an external device that is removed occasionally, Crashplan will just update the back up set when the device is plugged in.