Open the front face of the drive and verify that both drives have lights on them. If not then eject and re-seat them. Download the RAID configurator tool from our site here: DOWNLOAD
With the tool you can set the drive to RAID0 (all 12TB, no protection) or RAID1 (6TB, mirror) or JBOD (drives are individual).
After that has been configured you can erase the drive in Disk Utility so it can be used. If you are still having issues or it isn't showing the proper capacity then we would suggest returning your drive to the place of purchase.