I don't think the dual ATS configuration will fly. If you have 400A service, you need a 400A ATS.
Even if there are no code prohibitions against a dual ATS setup, an AHJ would likely look at such configuration with great contempt. If this is to be a separately derived system, such a set-up could not work since I don't know of any way a dual ATS setup could be interlocked.
Why can't you can't simply stick a 400A main between the meter pan and the ATS, if a service entrance rated ATS is not an option for some reason? Fused disco will be fine, but probably a lot more expensive and bulkier than a 400A main breaker. It's always a good idea to have a disconnect on the service side of the ATS anyway, in case the ATS needs to be opened for service. You'll likely be the one working on it anyway. Let's say a board or relay goes bad, servicing the switch now requires pulling the meter... cutting locks, seals, notifiying POCO, and so forth... who needs that? This is another reason why a disco should be on the genny side too, so that the ATS can't be re-engergized should the genset start-up.
As an added bonus to all this, anytime you have to work on the two 200A panels, they can be TOTALLY dead - no fear at all working on them. I've gotten zapped a few times by live lugs on typical services.
Joe
