You can build an array with those drives but even I would question doing this.
RAID 0 reserves space on all drives equal to the size of the smallest drive. The rest of the drive is not useable/lost.
Drive manufacturers use 1,000,000,000 as their gigabyte - 320 Gbytes is 320,000,000,000 bytes. Computer people use 1,073,741,824 bytes for a gigabyte.
320,000,000,000 divided by 1073741824 which is almost that 596Gbytes you see (the lost amount is due to the rounding up the manufacturers do also).
You are seeing the space RAID 0 created using 2 very different drives. If you want that space back either use 2 1TB drives or break apart that RAID 0 array and go back to native drives.