I am installing a 400 amp service (non-residential). Outside of the building I want to put a Combination Service Entrance Device (meter main) with two 200 amp breakers. These breakers will feed two 42 circuit load centers located inside and directly behind the CSED. Do I need main breakers in each of these load centers? I think it would be a good idea even if is not required.

Main breakers are not required in these panels fed by the actual Main Service Disconnecting Means, which are the two 200 amp Mains at the meter.

You may have Main breakers in the panels if desired for convenience and redundant protection, but not required by Code.

Just be sure to do the grounding of the neutral at the meter/disconnect combo, and use 4 wire feeder to both panels, and keep the neutral and equipment ground separate in both panels.