There is no need for each Pilot to see his counterpart's stick, period. Not if CRM, and a strictly no nonsense environment is followed.
in an ideal piloting environment I could perhaps be persuaded.
But when humans are involved, errors will happen.
PNF may have clued in had he seen his ss sitting on the back stop.
My underlying concern is that when stuff goes wrong, good information is needed for the crew to put things right.
Among other factors, ss position and some red on the altitude/VSI display might have alerted PNF even though it seems he was too nice a guy.
PNF does not seem to have had any positive ideas about what needed to be done -- nor did CDB.