California Code, Education Code - EDC § 22115.5

(a) “Concurrent retirement” entitles a member of the Defined Benefit Program to retire for service from the State Teachers' Retirement System and from at least one of the retirement systems with which the member has concurrent membership, as defined in Section 22115.2, on the same date or on different dates provided that the member does not perform creditable service subject to coverage under the other system or the Defined Benefit Program between the two retirement dates.

(b) A retired member who is subsequently employed in a position subject to membership in a public retirement system, specified in Section 22115.2, shall not be eligible for concurrent retirement.