The point is that, as safety-critical equipment, you can't fly the plane if one is broken. So you'd need to have two fail within a single flight, and fail in the same way, in order to cause an incorrect activation of MCAS. With just one sensor, it's much more likely.

Note that Airbus uses three of these sensors on their planes, so that when one fails you know which one it is, and can still rely on the signals from the two remaining good ones. Then you replace the failed sensor before the next flight.

This works until two sensors fail in the same direction:

>The aircraft's computers received conflicting information from the three angle of attack sensors. The aircraft computer system’s programming logic had been designed to reject one sensor value if it deviated significantly from the other two sensor values. In this specific case, this programming logic led to the rejection of the correct value from the one operative angle of attack sensor, and to the acceptance of the two consistent, but wrong, values from the two inoperative angle of attack sensors. This resulted in the system's stall protection functions responding incorrectly to the stall, making the situation worse, instead of better.

https://en.m.wikipedia.org/wiki/XL_Airways_Germany_Flight_88...

That was my original question. What is the probability of them failing at the same flight in the same way (say they got frozen at the same angle, hit by blizzard, etc, etc). Intuition is that the chances are high given their low MTBF.