No law imposing additional costs upon two or more cities or towns by the regulation of the compensation, hours, status, conditions or benefits of municipal employment shall be effective in any city or town until such law is accepted by vote or by the appropriation of money for such purposes, in the case of a city, by the city council in accordance with its charter, and in the case of a town, by a town meeting or town council, unless such law has been enacted by a two-thirds vote of each house of the general court present and voting thereon, or unless the general court, at the same session in which such law is enacted, has provided for the assumption by the commonwealth of such additional cost.