For Gods sake what harm could be done by linking the sidesticks so they move together
With non-linked SS PNF can hold the SS neutral (or rest his hand on SS pedestal), push the red button and direct take over in critical phases when required.
With linked SS, PNF should remove his hand to not obstruct the SS movement.
When it is required to take over, he now first have to visually locate the deflected position of the stick,
grab the stick and initially fight the other stick input.
(and not forget to push the button to disconnect the adjacent SS)
Valuable time is lost and forward facing philosophy interrupted in critical phases.
In AF447 linked SS could have contributed to another outcome but in the majority of airborne time it is completely useless and in critical phases it can be distracted or even dangerous (as described above)