You only need one guy to read a checklist and touch/verify what he's doing..
Challenge and response is really at it's core, about making sure something is getting done, brought to someone's attention and the item/thing is solved/acted upon, etc....by two people.
'Fl180, Altimeter check'
'Set to 30.12'
And both people watch it being set the correct setting.
That's the kind of redundancy that should happen in a crew cockpit...if one guy just reads the check list, never really looks up, or checks that the other is actually doing the item and correctly...you might as well be a single pilot op, it's just busy work.