50 ILCS 465/45

One or more Authorities (whether or not any of them are home rule units) may join together or cooperate with one another in the exercise, either jointly, on behalf of the Authorities, or otherwise, of any one or more of the powers conferred upon Authorities under this Act or other enabling acts or powers. The joint or cooperative action shall be taken only in accordance with and pursuant to a written agreement entered into between or among such cooperating parties.