When using H/W RAID the DSS V6 Lite recognizes only the 2TB unit that is presented to the system. However when using the S/W RAID DSS V6 Lite counts all the capacity from all connected hard drives. So to create 2TB LUN using S/W RAID you will need e.g. 2x2TB hard drives, which exceeds the DSS V6 lite 2TB storage limit.
You can use two 1TB hard drives and create 1TB RAID1 unit using S/W RAID.

BR,
Tomek