completely agree about this 'dual input' mode - crazy.
But also these modern planes seem to jump in and out of 'modes' without any warning or confirmation.
I believe the 2008 plane crash at Madrid (largely pilot error again) also suffered from some confusion about the plane 'mode'. It was set to 'normal flight mode' whilst attempting to take off (or something).
Again, both crashes largely due to 'usability' issues and cockpit design.
Madrid crash was due to a improper disconection of the flight-ground sensor by maintenance( following unclear documentation)That led to a take off without flaps ( the configuration alarm didn't sound as it should when the crew forgot to extend them). It didn't take off it only jumped out of the runway and then crashed.
It was not an usability matter just lots of small factors aligning, neither one on itself could have caused the accident.
There is a loud "dual input" voice + a red light just infront of you. Also if you push the red button on the sidestick and keep it pressed you override all te commands of the other one. This has several reasons, taking away the control from a disoriented pilot or disengaging a malfunctioning sidestick which is commanding random inputs.
At the very least, there should be a significant warning in the cockpit when "dual input" mode is active. That is an incredibly dangerous "feature."