Take advantage of exception number three to 230.40. It allows you to run two sets of service entrance conductors supplied from one service drop or lateral to supply a single family home and one detached structure.
230.40 Number of Service-Entrance Conductor Sets.
Each service drop or lateral shall supply only one set of service-entrance conductors.
Exception No. 3: A single-family dwelling unit and a separate structure shall be permitted to have one set of service-entrance conductors run to each from a single service drop or lateral.
You would have the new 320 amp service installed to a meter can that has provision for two sets of conductors on the load side. You then run two separate underground service entries with one serving each building. Since the service disconnecting means is located at each building you need make no change to the shop service equipment. You just run the new service entry to the house and wire it as a separate service.
A second approach would be to replace the service conductors to the shop or to parallel them with identical conductors of the same length. If those conductors were properly sized when installed then a second set of identical conductors run in parallel will raise the ampacity of the service entry conductors to 400 amps. You would then be able to mount a second 200 ampere service disconnecting means beside your shop panel to supply the feeder from there to the house. The feeder to the house would be four wire and the building disconnecting means and lighting and appliance panel boards at the house would be wired as feeder supplied panels that many in the craft call sub panels. Since the feeder to the house would be the "main feeder to a dwelling unit" it could be sized in accordance with
310.15 Ampacities for Conductors Rated 0–2000 Volts.
(B) Tables. Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.16 through Table 310.19 and Ampacity Table 310.20 through 310.23 as modified by (1) through (6).
(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders.
That would allow you to run 2/0 copper or 4/0 aluminum as the feeder from the shop to the house.
If the supply from the POCO to the meter is overhead you may want to install a grounding electrode system at the meter and connect the grounding electrode conductor to the neutral at the service head. This will tend to shunt lightning to earth rather than burning up your underground service entry conductors.
[This message has been edited by tdhorne (edited 11-24-2005).]