Denti: I'm not sure what you mean by 'background checked'. Was Lubitz 'background checked' and, if so, did it all impinge on his admittance to the cockpit? Obviously not. I'm a firm believer in the psycho-social factor of eyes...as in, you're less likely to act like a deviant if there are others watching you. Not foolproof, but there you go. In security, there's really no such thing as perfect. You just look for improvements. In my opinion 2 on FD is better than 1 on FD, even with imperfections.
Although that might be true, it now becomes extremely easy to reenact 9/11. Simply apply for a part time FA position, and you will be invited into the flightdeck with just one pilot there and a sharp axe right beside your jumpseat. Time will tell which is more likely, both are extremely remote possibilites.