In cases like this it's almost always some variation of normalization of deviance.

If company culture doesn't strictly enforce callouts 100% of the time, it's easy to get into a routine of everything being just fine 100's of times until it isn't.

Really these sorts of things show there should be a failsafe mechanism that provides tactile/haptic feedback to the pilots if the other inputs are "fighting" theirs.

Even an audible alarm isn't enough, as you'll likely get more than a few of those during events where it matters.

Proper CRM of course should make this "impossible" - but in high stress moments any more layers of swiss cheese you can add the better.

Agreed. There should be a stick-shaker or similar to let pilots know the other control is being used. Maybe not the existing stick-shaker, since that's usually for stalls, but something similar.

I have no idea why the pilot-monitoring would have taken his controls. Or why the pilot-flying wouldn't have called him out for it. Possibly a seniority issue - we've seen similar issues before. Hopefully the french aviation authorities can determine the root cause.

Ever see those really long fire trucks?

Two drivers

But the second driver of a tiller truck only steers the rear wheels, more like the pilot of a glider being towed.