When a plug sparks the mixture does not instantly explode, a flame travels out from the spark until all the mixture is burning. Now, when we have a large cylinder, that can take a little while, so the maximum pressure on the piston takes a while to build up, and at higher rpm, the piston is far enough down to reduce the effectiveness of the "bang". (This is why we advance the ignition timing with rpm). So two plugs firing at opposite sides of the combustion chamber will set off two flames that meet in the middle, so the whole mixture is alight in half the time, getting more combustion pressure when it can do the most good. As well as the time issues above, there is also a likelihood that as the single flame front moves across the combustion chamber that the increase in pressure and temperature can cause a compression ignition in the un-burnt mixture causing knocking. Single plug boxers like the R100 I used to have were very prone to knocking/pinking, and that is probably why BMW introduced twin plug heads. A "lost spark" in the other cylinder at the end of it's exhaust stroke is only there to simplify the electrics.
