2x1 tb drive

Well, the main difference is that a 2tb drive is one drive while 2x1tb drive is two drives. There are differences in individual drives. Some things to consider are spin speed, warranty, reliability, etc etc, depending on your specific need. I like to go by dollar per tb. When you break it down like that, a 2 tb drive can be a better deal than two 1tb drives, also, if you have two drives and one breaks down, you have a backup, with just one drive, if something goes wrong, you wait for a new drive to get there. Maybe I didn't understand the question.  

Well, one option is a 2TB drive, and the other option is two 1TB drives.