In a light piston twin, the second engine isn't there for safety. It's there for performance.
Yet another fallacy, we are on a roll.
Singles have been around that can match the performance of nearly every light twin for as long as I can remember.
Even today, you would have to go a long way to find a twin that can out perform a Mooney or even an SR22 in every practical sense. I can go faster in the SR22 than I can in either the 42 or an Aztec (two generations of twins in regular use), I can fly as high as makes no difference, and whilst I might get 200 feet more of climb performance out of the Aztec it as good as makes no difference.
I doubt the second engine was ever fitted for pure performance.
However passengers and regulatory authorities (and even some pilots) take comfort from the extra engine. The CAA has never approved single engine turbines (or anything else for that matter) under JAR-ops or before for IFR ops. Passengers are on the whole horrified when you let slip there is only the one fan.
Even the multi world started out with the 60 minute rule, that evolved into any transoceanic aircraft being required to have four engines. Whilst this has evolved again into ETOPS and will shortly evolve into LROPS the second engine is considered a prerequisite for safety, and even then, under ETOPS a second engine is still not quite considered as stacking the odds sufficiently in the passengers favour, hence the ETOPS requirements which are not, as many think, restricted to only transoceanic crossings even if in practice this is effectively the case.
No body wants to carry around an extra engine for performance, even if their was any truth in the fallacy, and everyone is desperate to see ETOPS evolve in LROPS but I don’t think even the commercial world will ever persuade the general public to get in an aircraft with one engine unless perhaps they think there is half a chance of landing it in a friendly field about the size of their back garden!!