Putting the sticks in opposite directions means the sticks are not in the same position. In the case of manual dual controls the sticks/wheels are chained together to ensure they are always synchronized. From the airbus literature I understand this is how their FBW sticks also work. The sticks are, however, also pressure sensitive. Is the "dual input warning" caused by pressure applied to the non-control stick.
To ask the question again; is there a setting/switch/button that makes only one stick the active one, or are both sticks always active?