I have an existing two-family home where the owner wants to upgrade the service and breakup branch circuits properly...lots of homeowner type wiring 'n shortcuts. Anyhow, there exists two meters outside and two separate main-breaker panels inside the basement. Right now there are two sisters living in the house, the sister who owns the house lives downstairs and has access to the basement and electric equip. through her apartment. When I asked the owner if she had given access to the basement to other tenants in the past, she replied "no." BTW, there is a door in the basement leading to the backyard, but she said she wouldn't give a key to a strange tenant, since the basement is finished and used as living space. With this in mind, as per the NEC I have to locate the second floor panel on the apartment, correct? Also, the service disconnect would either have to be located outside the house or in the respective apartment, correct?
I'll bet there's not a "hard and fast" answer to this one. A lot depends on the ordinances and regulation for existing buildings that are in force now for that duplex. The best source of information will be the electrical inspector that will eventually come to the dwelling to OK the power company reconnection to the upgraded service. The local power company meter placement requirements may also figure into the solution that will "sell" to the sisters.
Consider also, that the sisters, when faced with the cost estimate of relocating the rental unit service center, may find it much easier (and less costly) to give the tennant a key to the basement door.
I see this from time to time in the seven county metro area that I work in. The access is granted to the current tennant, the work gets done, inspected and completed. Then the next tennant doesn't get the key.
A lot of towns have "Property Maintance Codes" that address this issue. Check with your local BO. The new portions would have to meet current code. If no access, then the panel has to be moved to the apartment, or outside, where it is accessible to each tenant.