Seems as it the unit doesn't support any other RAID mode than 0. So the overall capacity is the disc size times 2 (2TB for 2 1TB drives etc.).
However, if you don't use the eSATA interface, a 2TB RAID0 is a waste of money since a single drive alone can saturate the other available interfaces.
If you need a larger volume (4TB or 6TB) it's ok, but for 2TB I recommend a single hard drive. As I said before it will be just as fast over FW or USB and has half the failure rate of a 2 drive RAID0.