I'm not sure I completely understand your situation so maybe I have misunderstood. The cheapest method to use and still ensure plenty of capacity is to simply use a 400A meter socket and feed out of it twice to two separate 200A panels. You would have to make sure you order lower lugs for the socket that allow two wires in each. Since nobody else has suggested this relatively cheap, simple remedy, I can only assume that I am misinterpreting your description.
