DFC:
If you look at the average (good) multicrew SOPs and you will generally find that during manual flight the PF has hands on the controls and the PM;
tunes the radios, sets the mode selects, arms modes, does everything with the FMS, crosschecks the navigation, fills in the PLOG and handles the communication.
With the exception of
underlined above, you will generally find that (good) multicrew SOPs require exactly the same of the PM when below FL100.
So if the PM (be it captain or F/O) is not able to turn a few knobs (probably only the altitude, since the PF is just following a magenta LNAV path as you say), I really would not like to be in the cockpit with him or her in an emergency!