The employees’ retirement system (or such components thereof as may be established by the governor or the governor’s agent pursuant to section 218(d)(6) of the Social Security Act) shall, for the purpose of this chapter, be deemed to constitute two retirement systems as provided in, and in accordance with, section 218(d)(6) one of which is composed of the members of the system who have expressed a desire to be covered under the Social Security Act and the other of which is composed of the members of the system who have not expressed a desire for such coverage. Upon request of the governing body of any political subdivision operating a retirement system, the membership of its retirement system may likewise be divided.