The obvious answer to this 'problem' is of course 3 man flight deck crews, but of course that will not or can't happen for a multitude of reasons.
True, but what can happen (and does happen with some operators) is that whenever a crew member leaves the cockpit for a toilet break, a flight attendant then comes in to the cockpit to accompany the remaining crew member. Although the flight attendant wouldn't be expected to take control at any stage, he/she can at least be there to unlock the door to allow the other fight deck crew back in to the cockpit. IMHO, this should be standard procedure to prevent something like this from happening again.