If the updated MCAS software needs two AOA sensors, doesn't that introduce a new single point of failure?
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.