Even with two pilots one goes out to toilet. I am not sure all airlines follow policy of bringing in a cabin crew for that, even that is just to open the door. But cabin crew who don't know flying cannot prevent a repeat of German wings because a 737 can be put on it's back in seconds. And if the reason for these rare occurrences is only due to second guy, you are suggesting that a great number of pilots are mentally disturbed and just waiting for an opportunity to kill themselves. Surely that's not the case.