The Building is only being served by one service with a single service disconnect and that happens to be on the outside of that building, per 270.70(A)(1). If this was on a pedestal out in the yard you might have a question but you are simply talking about distribution within the building.
IMHO

In fact you could run this inside with a single feeder and exploit 310.15(B)(6). I think you need to use 310.16 if it is 2 feeders.


Greg Fretwell