I have a 2600 sq. ft. home with four panels; two are used for an addition and are fed from one of the larger panels in a garage. (Did I ever mention I like fine control of circuits, and prefer "home-run" wiring.) The two main panels are 30 pole position panels and the addition's panels are each 20 position. Almost all positions are populated. I would, however, have to have a team of people doing different activities in parallel to actually use close to 200A.

In my configuration, the house side of the single meter goes to a combined generator transfer box and disconnection means (200A breaker). On the house side of the disconnection means are two feeders to two independent panels, each with its own main breaker. However, in this architecture, these breakers are not disconnection means as the term is specifically used, but panel shut-offs.

In NH, the power company owns the wiring to the house, and the meter. The drop, meter box, disconnecting means, panels, etc. are owned by the customer. This makes it difficult to exactly address the OP's question. I can address (roughly) what the code (NFPA 70) allows. Once past a meter and disconnection means for that meter, one can control how power is split in many ways. I suggest acquiring a copy of the NFPA 70 2017 Handbook and review Article 230 for concepts.

In many cases, multiple meter drops are allowed. If the power company controls the disconnection means and their protection current interrupters, then the potential current ratings will be determined by them, and not the customer. All the customer needs to do is properly protect feeders and circuits.

If I were doing this in NH, I could easily just set the meter boxes where appropriate and have two disconnecting means, downstream of which the appropriately sized cables could be run depending on the disconnection devices current ratings. Or I could have one service and put my own meter downstream and bill separately. (It would have to be an analog meter in this case.)

Does the OP's power company also supply the panels for the household circuits? I'm a little unclear about where the public service vs customer boundary is.

Why bother with separately metered service? just run a subpanel out there from your main 400A service.

That a good suggestion @branson4020 The main reason for considering separate meter is in the case ADU is rented out then tenant can pay electricity bills. Also more of the usage will be in the lower tiers of a Tiered-billing. I am strongly considering moving to a single meter.

@kaseki, the sub-panels are owned by homeowner whereas main-panel is owned by utility company.

@User no pool but the stovetop, microwaves, heat-pumps seem to add up very quickly. What is the typical oversubscription that is used?

A single dwelling may use service entry cabling rated (at the 75C of typical breaker panels) at 83% of the maximum load (20% oversubscription, if I understand your question). On the other hand, circuits should be configured to not need to supply more than 80% of their breaker rating (and 100% cable rating, although this itself is a calculation in many cases).

Public service power companies are excluded from the requirements of the National Electrical Code, but not from physics. Air drops from poles, for example, may seem undersized, but the conductor heating will be less in the air for a given current than conductors in conduit and cable jackets running in walls and in insulation.

For another meaning of oversubscription: When sizing the power needed for a household, the NEC provides a detailed description of calculation means. Get the Handbook. On the other hand, if one's loads are dominated by heat pumps, saunas, swim jets, electrically heated floors and driveways, multiple wall ovens, and other less common high power usages, actual calculation of the steady-state usage of existing and future equipment is better, in my opinion, particularly for an existing household. Note that most motor-driven systems use breakers significantly higher rated than their actual load, so actual loads should be used for such calculations.

200 amp service is IMO the min. now for a home there are so many things eating up electrical and if you want a induction stove that requires some serious electrical. I would do 400Amp use it for both units figure out a realistic price for usage and add it into the rent. Most renters actually like not having a power bill to deal with.