See Configuring Your ReadyNAS for Flex-RAID
You'll need to backup your data (if ReadyNAS is primary storage), do a factory default, in RAIDar click setup and choose Flex-RAID.
It's best if you delete the volume automatically created and create two RAID-0 volumes, one per disk. That way if one disk fails only the volume on that disk is lost.
Edit: Is this the new Duo v2? If so, you can select the Flex-RAID JBOD option to get two RAID-0 volumes. See the manuals: http://www.readynas.com/docs
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