The 8v engine has two plugs because the valves are that big that there is not enough room for the plug in the middle of the head. Just moving the plug to one side is not ideal as you can get an uneven burn. The best way round it is to fit two plugs, one either side, thus allowing you to fit valves like dinner plates and still get the fuel to burn evenly in the combustion chamber.
That makes sense. The 8 valve TS engine is only a TS to keep the idle emissions down and aid smoother running at low revs (read that somewhere).
The 16v is a TS to improve power output.
From what I've read, folklore sez that the twinspark (8 or 16v) is to improve power. The "real" reason is to ensure an efficient burn process (and therefore emissions), if that in turn equates to more power then all is good I think it's the 16 valves which produce more power due to the engines characteristics.