Imagine flipping a coin. Your chance of tails is 50%. What is your chance of a tails if you flip two coins? It's now 75% clearly (TT, HT, TH but not HH). Now i

Imagine flipping a coin. Your chance of tails is 50%. What is your chance of a tails if you flip two coins? It's now 75% clearly (TT, HT, TH but not HH). Now imagine it's not a coin but a normal distribution. If you sample from it twice then take the minimum of your samples, the chance that the minimum is below the mean is 75%. Just the same as with the coin but in another context. Obviously the time-before-failure is not normally distributed, nor are the sensors completely independent random variables. But the chance of failure of the system will be higher than one sensor, not double exactly but higher.