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)
