The only real solution or mitigator is to build the cockpit so neither pilot has to go beyond the cockpit door. That means making the fwd toilet part of the secure flight deck environment. Unfortunately for the pilots and pax that will cost money so it won't be done.
That won't be enough, as if the other pilot is out of its seat for a pee, he won't have time to come back quickly enough to avoid the suicide of the other pilot...
Anyway, even with another pilot in his seat, I think it is very easy for the other pilot to crash an airplane.
Moreover, if the rule is to have always two persons in the cockpit at anytime, the certification agencies should forbid immediately operations with only one pilot, as some transportations of the kind are authorized, the risk assumed being that
ANY pilot* MAY commit suicide!
The best way would be to give every pilot a pill so as to suicide alone avoiding to crash the airplane, and kill passengers, crew and some innocent other people on the ground.
Another good way would also to stop completely air transportation... No flight, no risk at all.
More seriously, as everything concerning safety for air transportation is based on probability of occurence of a failure -this is true for airfcraft and aircraft part manufactoring and also for medical rules fore the crews-,
the only question is to know what is the probability of occurence of such a "failure" and is this probability acceptable in terms of air transportation...
What is that probability?
And also, I think that imposing 2 persons at anytime is totally irrelevant for the case, but even with a very low probability, there should be some medical psychiatric and psychological and neurological thinktanks about a better detecting people -pilots and, why not, attendants*- who could have such behavioural tendencies.
*One could thing of an attendant -why not- who would enter the cockpit, kill both pilots, then lock the door and commit suicide... So, may be 2 or 3 is'nt enough and let suggest... 4 (at least!)?