The controls are mechanically linked. Unless the perpetrator incapacitated the other flier, it’s a test of strength. There’s no method for disconnecting the second set of controls in a 737.
That is incorrect. There is a friction clutch between the controls in case there is a mechanical jam the other pilot can at least control half the control surfaces. It needs a significant amount of force to detach, though. It is documented on Egypt Air 990 however that both elevator surfaces turned into opposite directions.