Whenever a zoning lot is divided by a boundary between districts with different density requirements, the maximum number of dwelling units or rooming units permitted on the zoning lot shall equal the sum of the maximum number of dwelling units or rooming units permitted for each portion of the zoning lot in accordance with the applicable district regulations. Such dwelling units or rooming units may be located wherever a building is permitted on the zoning lot. However, for portions of zoning lots in R1, R2, R3-1, R3A, R3X, R4-1, R4A, R4B or R5A Districts, no more than one or two dwelling units may be provided, as applicable.