Yes in JBOD it will present in your operating system as a separate mounted volume for each physical disk.
In any RAID configuration it will present as one mounted volume (in your case, roughly 12TB in size in RAID 0 and roughly 6TB in size for RAID 1) despite the number of physical disks.
In case you are not sure about the different RAID levels, you want to always stick to RAID 1 (mirrored) with these type of 2 - 4 disk NAS / USB RAID enclosure devices.
People usually buy these types of devices to store data that is very important to them. Considering this, there is no real point in buying one to keep in RAID 0, because if any of the disks die, all your data is gone from all of the disks. You might as well just buy 2 separate USB 3 drives. These types of devices only really make sense in RAID 1 where the mirrored data is providing fault tolerance. That way if one disk dies the unit and the data will still be accessible on the other disk, or if the unit dies as well, just take a working disk out of it and attach it to a computer with a very cheaply sourced SATA to USB adapter which you can get online for £10 or less.