How does the XP system work in co-op anyway? I would guess that only players who participate in combat get XP for slain enemies. If a co-op player enters a combat of their partner he should receive XP only from that point onwards. Is it like that?
And of course the second player out of combat shouldn't be able to hurt enemies in real time. Maybe it would be a solution to make the turn-based area much bigger but giving the second player double the action point in the first turn when entering the fight. That way he could run to help his co-op partner in turn-based mode, but without the change to do damage in real time.