Why did Airbus decide that a dual input that would double the input was a good idea?I have found after a number of years on the Bus that taking over whilst pressing the red thingy is not exactly instinctive.
Because in the worst case scenario where someone does something so mind-bogglingly stupid that an instinctive reaction from the other pilot happens without pushing the red button, the inputs are summed and go to zero, protecting the aircraft and giving a second or two to formally take control.
Equally, in say a GPWS or windshear situation, a tentative response from PF will rapidly become a full deflection input if PNF instinctively adds an input, safely giving the full performance capability of the AC (albeit in a messy way).
There's no alternative that "works". Making the Captain's sidestick always take priority in a dual-input scenario will only end in tears because the co-pilot that needs to intervene will be doing it instinctively to preserve his or her own life or the integrity of the aircraft, and will never push the take-over button. At my airline co-pilots aren't allowed to cover the controls so there's no chance of them getting their hand onto the sidestick AND pushing the red button.