Wow thats gonna be a fair size ATS eh!
If it were me for the "inside" part regardless if there is a generator upstream or not, I would come into a 400 A disconnect then out to a splitter trough and out of that with 2 x 200A fused disconnects or breakers. Whatever you prefer.. That way with the trough it allows you to "expand" a bit should the homeowner want to add something down the road such as a " shop" or something along those lines.
Just out of curiousity, I do not know what the codes are in your area but should there not be some sort of disconnect between the meter and ATS?
A.D
