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Other engines have had dual spark plugs. My wife's V8 Benz has 16 spark plugs--talk about a royal pain to change!
I know that in the aircooled Porsche world, it is used to allow higher compression than with the single plug setup. The second plug is placed symmetrically with the original plug. The general rule of thumb is that you can run a full point compression more on the same fuel with twin plugs as with single.
That is highly dependent on combustion chamber shape, though. The wedge-shaped aircooled VW heads benefit much less than the somewhat-more-hemispherical 911 combustion chambers.
One friend of mine remarked that dual spark plugs are a band-aid to help suboptimal combustion chamber designs run more compression... I'm not sure he's wrong!
In other applications, such as aircraft, the dual plugs are there only for redundancy. They are typically run by completely separate ignition systems such that if one component fails there is a separate system still doing the work.
-soD
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