Mich. Comp. Laws § 38.1669

Sec. 69. A qualified participant shall not participate in any other public sector retirement benefits plan for simultaneous service rendered to the same public sector employer. Except as otherwise provided in this act or by the department, this section does not prohibit a qualified participant from participating in a retirement plan established by this state or other public sector employer under the internal revenue code. For the purposes of this section, "public sector employer" includes, but is not limited to, a reporting unit.