If the updated MCAS software needs two AOA sensors, doesn't that introduce a new single point of failure? Regarding the 737 MAX story, the New York Times writes:


  "Boeing’s software update would require the system to rely on two
  sensors, rather than just one, and would not be triggered if the
  sensors disagreed by a certain amount, according to the three people.
  Given that the 737 Max has had both sensors already, many pilots and
  safety officials have questioned why the system was designed to rely
  on a single sensor, creating, in effect, one point of failure [emphasis mine]"


Now I understand that this avoids a False Positive, when one erroneous sensor triggers the MCAS.

But, considering the opposite situation, doesn't this update introduce a new single point of failure, a False Negative, when a stall should be counteracted with MCAS but it isn't, because only one sensor detects it?

(Or if not, what am I missing here? Is it that a faulty sensor fails in a certain way and will not read normal AOA erroneously?)
Noting that the details of the MCAS update are yet to be publicly confirmed by Boeing - no I don't believe you are missing anything. 

MCAS was meant to be a system that only kicked in when the pilots were letting the situation get out of hand. It was to aid in stall prevention, but does not do anything the pilots can't (as long as their situational awareness would allow). In a million flights MCAS would not be used once unless there were other serious difficulties at play. 

In such a system it is much better to have a false negative than a false positive. A false negative means that the aircraft doesn't change anything, and continues to follow the pilot commands. A false positive means... well, it looks like there are 2 crashes that demonstrate what happens.