Tiesco, the reason you need two contact breakers - unless you go to electronic but the principle is the same - is because the points are mounted on the camshaft. Which turns at half of crankshaft speed - because this is a 4-stroke engine. As Pete said, the pistons are rising and falling together but while one is on compression - ie valves shut - the other is on exhaust. The camshaft achieves this in half a revolution. During the next 180°, it reverses the opening and closing sequence, while the crankshaft and pistons perform another full 360° revolution. So you need to fire at least one spark every half-turn of the camshaft. Hence two contact breakers, or two pick-ups for an electronic system with sender plate on the cam, as with my Boyer.
Using one coil, as on my XS, means you fire both plugs together - this is the wasted spark set-up, because one cylinder is on the exhaust stroke as Pete said.
When Yamaha went to CDI, they used a pickup on the crank, so only one is needed.
You are right, this is interesting stuff, when I'm working on an an engine I just love seeing how it all works.