Common sense dictates in the case of one engine inoperative on automatic pilot, the PNF should operate the AFDS.
If the PF is flying manually and the PM makes a selection on the MCP the PF can decide no to follow that Guidance (which is what makes him the PF).
However if the autopilot is engaged and the PM makes changes to the MCP then he automatically becomes the PF because he is directly "PF-ing" the trajectory of the Aircraft.