65 ILCS 5/11-74.5-11

One or more municipalities (whether or not any of them are home rule units) may join together or cooperate with one another in the exercise, either jointly or otherwise, of any one or more of the powers conferred upon municipalities under this Division or other enabling acts or powers. Such 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.