40 ILCS 5/8-231
Any participant in the fund who becomes a participant in any other annuity and benefit fund, annuity and retirement fund or any pension fund now or hereafter in operation in the city for the benefit of employees of the employer, may elect to receive a refund or annuity from this fund in the same manner as he would if he then resigned from his position in the service and had not become a participant in such other fund. No credit is allowed for any period of service as a participant in this fund for which the employee receives credit in such other fund, and no annuity shall be paid to such participant by this fund while he holds a position in the service which entitles him to participation in such other fund.
If a participant in this fund is employed concurrently by an employer whose employees are participants in a public retirement system created under other Articles of the Illinois Pension Code as well as by the employer as defined in this Article, any earnings from such other employer during such period of concurrent employment shall not be considered for annuity or benefit purposes under this Article.
40 ILCS 5/8-231