Art 100 defintions:
Dwelling Unit. A single unit, providing complete and independent living facilities for one or more persons, including permanent provisions for living, sleeping, cooking, and sanitation
If that defintion is met it is a dwelling unit no matter what else is located in the same building. That also means all other dwelling unit requirements must be met such as 210.52, GFCI and AFCI requirements,
If this space does not have permanent provisions for cooking, or sleeping, or has no bathroom within the unit then it is not a dwelling unit. It is a dormitory room, office, or other space.
I have a 400 amp meter combo to two 200 amp panel approx 30' away....The wiring for this is located in the deans apartment which is physically attached to the dormitory. Only the second panel box has wiring that leaves this apartment and enters the dormitory.
Only the deans apartment is potentially a dwelling unit, the rest of the building is not. The two feeders you mention can not use Table 310.15(B)(6) because they are not feeding "all loads that are part or associated with the dwelling unit." as is mentioned in 310.15(B)(6). If you had a feeder supplying the dwelling unit only and it supplies all of the dwelling unit load then you can use table 310.15(B)(6) to size that feeder.
It may be likely to have higher heating,cooling, and lighting loads than a residence.
I think it is more likely to require less heating and cooling than a stand alone residence of the same size. I'm talking the dwelling unit itself and not the entire building. For starters there will be likely be conditioned spaces adjacent to it at least on one side maybe even above or below. That means there is less exterior wall, floor, roof, and even windows and doors which are the biggest heat loss culprits, to lose conditioned air to as compared to a stand alone residence.