Hi Check Airman,
The use of only 1 FD whilst the PF is handflying FD OFF can be quite misleading. You will be most of the time out of the bars as the sensitivity of the machine regarding corrections is obviously much higher than the human's one. I believe that the PNF has already his job in monitoring the parameters regardless of the FD indications. You can't monitor something that the other pilot is not following, i.e. FDs orders.