- properly knowing what to do in case of sensors disagreement could be almost untractable
I bet when the plane is actively trying to crash itself into the earth from less than 1000 feet, you would be willing to try the other one. I mean, the pilots were surely aware that the current setup was not working, correctly, even if they didn't have enough time to fully pinpoint the problem. Even without time to check out the "backup system," you would probably want to go with the odds of both "redundant" systems going screwy being relatively tiny. Esp when the odds of surviving the impending crash are going to be pretty close to nil.
This is like having a backup parachute that you don't pull, just in case it doesn't work. Well it's even worse. It's alternating parachutes on each jump, but when the one doesn't work, you give up on parachutes, altogether. Even though you carry two, and the other one worked fine on the previous jump.*
The arrangement they have made seems to be the opposite of redundancy. And is simply a doubling of the potential failure points.
*Boeing and/or FAA might not have considered the flight computer and AOA sensor (and w/e other stuff they are cleverly alternating/testing) to be as critical as a parachute. The plane may be very safe to fly without any of that, under most circumstances. But... whey carry and alternate between two "redundant" systems if you don't make them redundant?
edit: I suppose after regaining manual control of the plane, perhaps the pilots would attempt to boot up the backup computer/sensor? And having 2 AOA sensors, one would be able to know there's a disagree, even if an adverse event hadn't occurred, yet? I can see why using 2 AOA sensors will be helpful. But what's the point of alternating which one is the primary and which is the "ignored one?" i'd think after a sensor replacement/maintenance, you would set that newly changed sensor as the 2nd stringer, until it proved itself. And until the tried and true one started getting closer to pasture, regarding wear/age. Or.... just leave the left one primary, all the time! If someone does not notice the disagree (because they didn't spring for that upgrade, maybe?) you are ensuring they will be using the malfunctioning sensor on the very next flight, just in case they got lucky and it was on the secondary on this one. If you make the left one the primary, the right could fail and you could never even notice it (until you sprung for the upgrade warning light?). Even if the left/primary failed, you wouldn't switch to the other one, anyway, apparently, so you wouldn't even notice that your backup chute had a giant hole in it, for the past 3 years.
In summary, if you're not going to use the other sensor, you're not going to observe or act on the data from that sensor, then it's better, statistically, to have just 1 sensor than alternating between 2. You're just guaranteeing that if 1 of the 2 fails, you will be using that one for sure. And even if you're doing all of the above, diligently, I still see no tangible benefit from alternating which one is the primary. If there is any way to determine which of the two sensors is more likely to fail**, then alternating would have tangible cons, though.
** e.g., maybe one has worked perfectly fine for the last 1000 flights and is still in the prime of its service life, and the other one just got replaced, yesterday. I might have a slight preference, if I were to bet on one. But maybe this sensor has been through enough testing that there's an MTF graph to check, first, before I place any big money.