Two magnetos?
Firstly: redundancy. A magneto is a completely self contained ignition system, complex and at least in the early days, not very reliable. It doesn't weigh very much or in the context of a whole aircraft, cost much either.
Secondly, two magnetos allow two plugs per cylinder. In a large cylinder there are real advantages in having the flame front propagate from two spark plugs. Plugs themselves were not very reliable in the early days so redundancy there was useful also.
Thirdly, having two entirely independent systems allows one system to be turned off so as to test the performance of the other. ("Mag drops")
It is the electrical side of the magneto which is considered less reliable. Whilst the drive can, and has failed, most mag. failures will be electrical. The Griffon magneto had two completely separate magnetic and electrical circuits within one unit. If either should fail, the other worked normally.