It needs three AoA sensors, like Airbus planes have. Two isn't enough for such a safety-critical system. When one malfunctions, you still need to know what the correct value is.
On Lufthansa 1829, two of the three AOA sensors on an A321 erroneously agreed, causing a system similar to MCAS to kick in[1]
The sensors can fail due to being frozen, or also be damaged by birds traveling in flocks
[1] Third bullet point under the A321 heading: https://en.wikipedia.org/wiki/List_of_accidents_and_incident...
It would be ideal to use different technologies for the AoA sensors to help minimize common-mode failures. Integration and crosschecking against other systems (IMU, radar altimiter, DGPS, an open-loop model of the plane based on engine settings and control surface positions, etc) could give the pilots and flight envelope protection systems a better understanding of their situation. Also, the signature of a failed/frozen vane-type AoA sensor is pretty obvious and I'm surprised the software doesn't take that into account.
Good writeup on LH-1829: http://avherald.com/h?article=47d74074