The prefix co- implies that the person is equally a qualified pilot. But is not the captain or, if you prefer, commander.
The PF/PNF, handling-non-landing, CM1/CM2 rubbish just fudges the issue.
'First officer' is a far more subservient term and doesn't even indicate that the person is a qualified pilot.
'Second pilot' once meant 'another pilot' - but was viewed as 'second class' pilot. Wrongly.