It sounds like you are planning to feed this dwelling with two 200 amp feeders. You should install one 400 amp feeder to the dwelling using either 400 CU or 600 AL. At the dwelling you can install two 200 amp MB panels next to each other or a single 400 amp panel.
225.30 Number of Supplies
Where more than one building or other structure is on the same property and under single management, each additional building or other structure that is served by a branch circuit or feeder on the load side of the service disconnecting means shall be supplied by only one feeder or branch circuit unless permitted in 225.30(A) through (E). For the purpose of this section, a multiwire branch circuit shall be considered a single circuit.
Curt
