94:4 Apartments that are built on an upper floor with a balcony in front, which one uses to access the courtyard via stairs, and from the courtyard to the public domain, is like the case of two courtyards, one inside the other. The balcony in this case is like the inner courtyard.
94:5 A house that has two apartments, with a corridor that one would use to access both of them, and the two owners live in the two apartments, it is forbidden to carry even from an apartment to the corridor. Similarly, even if there is only one apartment that is divided into two rooms with two people living in them, even though the inner room only has a door leading to the outer room, and the outer room only leads to the courtyard, it is nevertheless prohibited to carry even from one room to the other unless they make an eiruv. The case of a person who owns the two houses or apartments and rents or lends one of them to a friend is discussed in Orach Chaim 370.