How and why can multiple people control the Boeing B-29?
In aircraft of this vintage, all the controls are mechanically interconnected.  If the left-hand pilot's control yoke/ wheel moves, so does the right-hand pilot's.  Likewise for the rudder pedals.  And if a pilot or the flight engineer moves a throttle lever, the other throttle lever for that engine will also move.
If several people are exerting different inputs on the same control, then-- &quot;strongest man wins!&quot;
But normally the crew would have an understanding, at any given time, as to which pilot would be flying the plane, and who (which pilot, or the flight engineer) would be controlling the throttles.  Standard protocols would be established, and reinforced via training.  Normally only one pilot would be &quot;flying&quot;, i.e. handling the primary aerodynamic controls (aileron, elevators, and rudder, via the control wheel and the rudder pedals), at any given time.  Normally the flight engineer would handle the throttles on this aircraft, in accordance with whatever requests the pilot flying the plane made via the intercom.