Any person who has pension credit in 2 or more participating systems shall be entitled to a proportional retirement annuity, and his survivors shall be entitled to a survivors annuity in accordance with the provisions of this Article, if his combined pension credit is at least equal to the longest minimum qualifying period prescribed by any of such systems. The qualifying period of each of these systems shall be that which was in effect on the date of the employee’s latest withdrawal from service covered by any of these systems.
     Each participating system shall, in determining eligibility for a proportional retirement annuity or survivors annuity, consider the combined service and contributions of the employee for which pension credit has been granted under all participating systems. If the law governing a participating system provides that a retirement or survivors annuity shall be payable only if the annuity exceeds a certain dollar minimum, the proportional annuity provided by pension credits under all participating systems shall be considered in determining whether this requirement has been met.