I recall when I was a CFIIMEI I always mentioned the reason there were two magnetos.
there were two reasons as discussed by the porche driver above.
I also recall later on how easily the plugs fouled on certain types of 100 octane gas and one set always seemed to foul worse than the other in certain lycoming engines.
one funny story...I recall how a magneto had failed and was removed from one of our planes...a different mechanic was told to fix it and he said...which one? the answer was the one that was ''missing''.
well, he started the plane up to see which mag missed during a runup
but the MAGNETO WAS PHYSCIALLY MISSING FROM THE PLANE
what an awfuol oily mess!!!! be careful what you say.