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, wherever portions of a zoning lot are limited to single- or two-family residences pursuant to Section 22-12 (Use Group II – Residences), inclusive, no more than one or two dwelling units may be provided, as applicable.