It depends on how you have the property listed. If you have it listed as one single property (for example 123 Main Street Unit A&B City, state, zip) then you would assign the expenses as one unit especially if the property is deeded as a single parcel. If you list the property as 2 different properties (for example 123-A Main Street and 123-B Main Street) then I would recommend that you separate the expenses and assign the expenses accordingly.