Mich. Comp. Laws § 46.187

Sec. 17. Any 2 or more adjoining counties which have, by resolution of their respective boards of supervisors, authorized and directed the establishment of any of the improvements, facilities or services authorized by this act, may contract for the joint establishment, operation or maintenance of any such improvements, facilities or services, or any portion thereof. Such contract shall provide for the establishment of an administrative agency to be composed from the membership of the respective county agencies, and such administrative agency shall have and exercise all the powers and duties conferred upon a county agency under the provisions of this act, except as the same may be specifically limited by the provisions of said contract. Any bonds issued to finance the construction of improvements under such contract shall be the joint obligation of all participating counties.