The panel is one panel. The gist of what happened is that it took a while before the electronics used were able to do 4K from one data stream using just one display controller chip, so MST (multi-stream transport) was used so that two chips could be used to drive the physical display.
Ref: YouTube video: NewEgg ASUS PQ321 4K 31.5" LED LCD IGZO Monitor Overview & Interview at about 3:20.