The amount of times you hear of a pilot coming back to the cockpit and finding his colleague asleep is scary, but it almost never ends up in catastrophe as planes pretty much fly themselves.
I'm not sure that insisting on having two people in the cockpit at once would help. Indeed, looking at this it appears that the crash might be an unintended consequence of insisting that cockpit doors remain locked - i.e. this might not have happened if it weren't for the 9-11 inspired safety regulations.