40 ILCS 5/22-405

Employees of two or more municipal corporations having the same territorial limits and the same taxpayers, or of two or more bodies politic and corporate having the same territorial limits, may be included in and become members of any pension fund operating in any one of such municipal corporations, or of such bodies politic and corporate, and shall be entitled to all the benefits of such pension fund, whenever the terms and provisions of the law creating such pension fund shall so provide.