§ 18. Administration of civil service in jointly established agency. 1. Except where otherwise expressly provided by statute, when a public agency is established and maintained jointly by two or more civil divisions, including school districts, in the same county, the provisions of this chapter shall be administered for such agency by the municipal commission of such county, unless the appointing authority of such agency shall elect, within sixty days following the establishment thereof and with the approval of the civil service commission of a city, if there be one, participating in the establishment and maintenance of such agency, to come under the jurisdiction of the civil service commission of such city.