Whatever happened to the " I have control" -- "You have control" system that I grew up with in a bygone era of aviation?
Having only one pilot on the controls avoids any confusion about who is doing what.
Usually, simultaneously with the vocal command and acknowledgment of a transfer of control, a slight shake of the control stick/yoke is also made by the pilot assuming control of the aircraft in order to confirm the transfer. In cases where an immediate transfer of control was necessary there was no question who had the aircraft just from the level of force being exerted by the pilot taking control.
Obviously, with independent, non-connected "joysticks" this tactile feedback would be unavailable.