Original idea was to allow use of bigger diameter valves by moving spark plug from top of combustion chamber to less centered position. To ensure even combustion regardless of that was spark plugs doubled so that they could be placed symmetrically. Both spark plugs were similar on 8 valve engines and they were fired simultaneously
On 16 valve engine is different anyhow. There is one big plug and another smaller one. Bigger spark plugs is placed on top of combustion chamber and smaller to side of it. Some say they are fired simultaneously some say that smaller is fired after bigger one to complete burning. The meaning of second plug is not anyhow as critical on 16-valve engine as on 8 valve. Some say that smaller plugs are added just for that reason that we could still use twinspark badge on Alfas.
They fire simultaneusly. Every cylinder has its own ignition coil, connected directly to one plug and via a cable to the plug of another cylinder. Cylinder 1 & 4 fire at the same time, and 2 & 3 fire at the same time. I think the idea is that even if a plug or a coil breaks down, the car will still be running on 4 cylinders.