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The meaningful question in the absence of data is "what are the odds of getting five heads in a row?" or in this case the question I asked about both failing in a short time span.
These are classed as random events; if, under a given set of circumstances the probability of each one failing is X, the probability of both failing is X provided there is no interdependency, i.e that the probability of failure of either is not affected by the failure of the other.