2nd breaker
2nd breaker
I'm not sure if I understand the question, however, if you have 400 amp conductors protected by a 400 amp breaker, those conductors can feed as many "panels" as you wish. The second panel would not need a breaker as long as the conductors were 400 amp. This is not ment to address "disconnect" issues in regard to the 2nd panel, but merely addressing overcurrent protection.
Does that answer your question ?