The question is more, I would say, if I may, what the risk is that you want to mitigate with this solution. If you anticipate that you will have a hijacker on board who will bypass all of the security regimes installed in all the airports, that those security people will not identify a possible hijacker, as happened on 9/11, and there is an information that on a certain flight a certain assassin will be able to infiltrate the airplane, then this could be the answer.
It could not be the answer, obviously, for mass production. On every flight, you could not have the first two rows of any airplane manned by air marshals.
It is related to and based on specific intelligence information and of course a lot of training. It's very risky.