Two or more local authorities may, with the sanction of the Board, combine together for the purpose of executing or acquiring an interest in or maintaining any works by this Act or any other Act authorised in regard to water supply that may be for the benefit of their respective districts; and ail moneys which they may agree to contribute for the execution or acquisition or maintenance of such common works shall, in the case of each local authority, be deemed to be expenses incurred by them in the execution, acquisition, or maintenance of works within their district.