The last few 320A/400 Services I've done have been 2 - 200A cb's as feeder distribution, as "all in one meter main". Conductors sound right, but confirm it... Assuming this is a single occupancy residence... Check 310.15.6B if allowed in your area. Some areas with lots of AC don't allow this under sizing.
Our Power Company doesn't like up-grades of this much without checking the engineering for the neighborhood first. Most of thier transformers went in in the 60's, and are under-sized by todays standards.
230.7 I don't believe will be an issue as they are both feeders, if you used a meter/main! If not, you would have unprotected service conductors though a panel, not cool!
Still either way, I would pull the conductors to a can, then the panels. Or go direct.
But the meter/main combo is the way to go! No grounding/bonding nightmares, and once you and a friend lift it into place, it's done. (The last one was 4'X 5'tall, and wieghed more than I did....)
