Alfa Romeo brought out a 'Twin Spark' engine in the 1980s. At the Birmingham Motor Show I asked one of their engineers on the stand what the reason was, and he said it was so they could get bigger valves in. In this case the diameter of each valve was almost the radius of the cylinder. Offsetting the spark plug would reduce the efficiency, but adding a second one facing it restored it.
A friend of mine has a Mosquito powered hang glider harness with a Radne Racket two stroke engine. He added a second plug to this to help with inflight re-starts. The usual plug runs off the flywheel magneto, whilst the second uses a battery and coil which does not need a high RPM for a good spark.
I believe there is a possibility that if you have two spark plugs, if one fouls but the cylinder keeps firing on the other plug, the first will sometimes clear itself, whereas it is unlikely to do so if the cylinder is just pumping without combustion. Could anyone confirm this?