How to make RAID from USB drives
RAID stands for redundant array of inexpensive disks. It combines multiple disk units into a logical unit. If you make a RAID with two 128GB USB flash drives, it will appear as a single 256GB drive. In theory, if you had a single program that requires 256GB, you would have enough space using RAIDs from two USB drives, whereas if you used two separate disks and did not merge them into a logical unit, you would be limited to 128GB.
I like the step by step instructions on this eHow page for detailed instructions on how to build RAID from USB drives.