Let me try again on this point: 400A for a single family is excessive.
Very excessive. It is perfectly legal, and reasonable, to have (say) a
200A main panel, with a bunch of breakers feeding part of the house,
and also feeding two 100A subpanels, each also with breakers feeding
other parts of the house. Just because you are feeding two 100A
subpanels does NOT mean that you need to have a 400A main panel.

In multi-family dwelling mode, 200A panel, 100A panel, and 100A panel
on three separate meters should be fine. In single family mode, a 200A
panel on a meter should be plenty, and it can just feed the two 100A
subpanels. You would probably want to feed the 100A subpanels with a
wire and a breaker rated for at least 60A, or maybe even 80A (if you
are expecting some enormous power consumption on one particular floor).
Really, 60A is a hell of a lot of power for an upstairs in a house.
Even a big house.

So my point is: the 100A rating of the subpanel means _nothing_ when it
is being used as a subpanel. The upstream 60A breaker (say) in the main
panel will keep the subpanel from overloading, and it will effectively
turn into a 60A subpanel. And you can still put a zillion amps worth of
breakers in that 60A box if you like, just as you can feed a zillion
subpanels of whatever capacity you like from your single 200A main
panel. The numbers do not need to add up, and normally don't (I have a
100A service, and at least 150A worth of breakers in my main panel).

Of course, you have to make sure you can find a 200A panel with enough
slots, and that can take two 60A or greater breakers in addition to the
rest of the breakers in there.

-Kevin