If you change the pressure setting on your side because the other guy is out of the room or whatever, it will fly according to the onside FD so even if you've got STD on your side and QNH on the other, the autopilot will do it's job with the associated AP. Technically, i'd imagine that TCAS should probably also be swapped over if you change control as the TCAS will fly according to the pressure setting selected on the associated FD, although perhaps it works in STD at all times to avoid altimetry errors, i don't know.