You can use Flex-RAID and create two RAID-1 volumes however the ReadyNAS will only backup to Volume C. You can create up to four volumes, but any given disk can only be included in two of these.
Also note that a RAID-1 volume requires two drives. A single drive volume would be RAID-0 as there is no redundancy.
The ReadyNAS limits the size of Time Machine backups by setting a quota (see http://www.readynas.com/TimeMachine
) which is shared between all your Macs Time Machine backups. The backups are stored on Volume C. On Sparc ReadyNAS (e.g. Duo, NV+) you can set a limit up to 2TB and on x86 ReadyNAS (e.g. Ultra, Pro) I think you can set whatever limit you want now.
The use of a quota is much simpler than trying to use multiple volumes and allows you to grow the space allocated to Time Machine independently of your drive upgrades.
I would recommend you purchase a ReadyNAS Ultra or ReadyNAS Pro. These support 3TB drives, have fast Intel CPUs and are much faster than Sparc ReadyNAS (have which have slow Infrant Sparc CPUs). x86 ReadyNAS (e.g. Ultra, Pro) are much more future proof.
Also note that NetGear ReadyNAS have the best AFP and Time Machine support: http://www.netafp.com/status-of-netatalk-and-afp-support-by-nas-vendor-322/
Whilst there is no support for Time Machine backups from Macs running the soon to be released Mac OS X 10.7 Lion yet, NetGear being a funder of Netatalk development is a clear indication that they intend to provide this support.