I agree with glueball. There is always a designated captain. Only trouble is, today I flew with another training captain and neither of us had enough hands on practice on the FMS to use it properly and take less than many goes to get it right!
I revise my earlier statement and now think a descent co-pilot should always be carried if only to work the FMS.
However I still think the most valuable flight deck member is a proper hairy old flight engineer!
Looking at the past though rose tinted specs.
MM