((a)) ** .—** A State government may provide by law for the allocation of amounts among units of general local government in the State on the basis of population multiplied by the general tax effort factors or income gaps of the units of general local government determined under sections 6705(a) and (b)  or a combination of those factors. A State government providing for a variation of an allocation formula provided under sections 6705(a) and (b)  shall notify the Secretary of the variation by the 30th day before the beginning of the first payment period in which the variation applies. A variation shall—

((1)) provide for allocating the total amount allocated under sections 6705(a) and (b);  and

((2)) apply uniformly in the State.

((b)) ** .—** A variation by a State government under this section may apply only if the Secretary certifies that the variation complies with this section. The Secretary may certify a variation only if the Secretary is notified of the variation at least 30 days before the first payment period in which the variation applies.