Each SpokePOV board has LEDs on both sides, so it shows on both sides of the bike.
The reason for using more than one per wheel has to do with how fast you want to ride. At slower speeds, the image from a single SpokePOV won't 'fuse' very well. With one, you want to be doing at least 15mph. Two for 10mph. Three for 7mph.
The board is quite heavy - I'm not sure I'd want to be doing 15+mph with a single, unbalanced SpokePOV. With 2 or 3 units, you can balance them out.