It didn't fail to tell him. That's what the dual input alarm is for.

Now if that alarm was raised then that suggests both pilots were in error as they both seemed to have ignore that alarm.

However that was never the reason for my original reply.

All I was replying to was idea that the flight control was doing some sort of averaging of the two pilot inputs.

That to me just seems illogical as it would asume two pilots trying to fly the same plane and that scenario will never end well.

The sounding of the alarm is in the transcripts in the accident report.

The Airbus does average the inputs if both pilots are making inputs at the same time.

Do you have a url/link/pdf that describes this behaviour?

"When both sticks are moved simultaneously, the system adds the signals of both pilots algebraically."

In particular it also says this:

To avoid both signals being added by the system, a priority P/B is provided on each stick. By pressing this button, a pilot may cancel the inputs of the other pilot.

From this link: https://en.wikipedia.org/wiki/Air_France_Flight_447

As pilot flying, Bonin took control of the aircraft via the side stick priority button and said, "I have the controls."

Which again suggests at least in this case, Bonin was the only one flying the aircraft.