One other note. Besides early boarding having the Marshals sit together would seem to be a tip off and a weakness in that a single shooter would have both in his sites at the same time.
I was watching a cleanup crew coming out of an airplane while i was waiting for my own plane, and i wondered the same thing, Why isn't there someone supervising these people?
Good point. Also it makes it hard to keep an eye on other parts of the cabin.
I also recall reading the the marshalls have a dress code.