If I have underground utility service to a 400 amp meter/main/dist service (such as Eaton HP40SH) mounted on the exterior of a residence in which there is distribution to 2-200 amp panels that are mounted indoors back-to-back with the service and protected by 200 amp circuit breakers at the outdoor panel, have I violated 225.30 by bringing multiple feeders to the building?

The Building is only being served by one service with a single service disconnect and that happens to be on the outside of that building, per 270.70(A)(1). If this was on a pedestal out in the yard you might have a question but you are simply talking about distribution within the building.
IMHO

In fact you could run this inside with a single feeder and exploit 310.15(B)(6). I think you need to use 310.16 if it is 2 feeders.

I agree with Greg. This is a common install here, using a 320/400 amp meter base, and two (2) 4/0 feeders into two panels w/200 amp MOCP back to back with the meter pan.

The exterior main is required when the panels are not 'back-to-back', or the client/EC opt to install it.

(Note 270.70(A)(1) in Gregs post is actually 230.70(A)(1)