One guy standing can,with a little bit of effort,neutralise one guy who is seated.Even with 2 pilots in the flightdeck,if one of them makes a certain choice then the result can still be the same.
Yes, but it would still be a much, much higer hurdle than just waiting until the other left the Cockpit and then start pursuing what one's up to.
Suicides are rarely direct acts of violence towards others and the People committing them are seldom extrovert, physically violent People. If anything they only tend to be violent by effecting 'collateral' fatalities as is the case with quite a number of wrong-way Drivers.
So, such a measure would probably massively reduce the risk of such an occurence although probably indeed not totally eliminate it.
The other Thing is that the door locking logic might need some tweaking, especially in the light of the fact that after 9/11 passengers will not sit there and wait when an obvious intruder is threatening a Crew member in order to enforce antrance to the Cockpit. So, a code that after -say 30s- allows entrance no matter what, would allow passengers/CC to intervene should a Cockpit Crew member be threatened to allow entrance to the Cockpit and still prevent this really tragic and horrible Scenario.