I think it should be 'Basic disks', but I'm not very good in marketing names.
Anyway, you can simply try it. If you can create 2 disk-size volumes having only two disks, you've chosen an appropriate layout. If it doesn't work, simply delete the volume and try another layout.
Under the hood it's all the same. ZyXEL always uses some raid setup. For a single disk volume you have the choice between a single disk linear array (disks can be glued after each other. Sometimes this is called JBOD, but not always. Sometimes JBOD means 'one volume per disk') or a single disk raid1 (in which case you can add redundancy later, but that's not mandatory). Both are usable as a 'one disk per volume' setup. There is no difference in performance. Theoretically you can change from one to the other without touching the embedded filesystem, but I don't know if the firmware supports that.
Would this procedure cause any problems?
It shouldn't. But as said, I advice to remove the existing disks before setting up new volumes. After that you can put the disks back, and they will be recognized. (Maybe you'll have to re-enable the shares). Better safe than sorry.