Upon concurrence of the parties or upon the submission of a joint request, a member of the agency staff shall be assigned as grievance arbitrator. The parties shall not be permitted to select a grievance arbitrator from a list of agency staff members, or to exercise a right of rejection on appointments made under this section; but may jointly express a preference for appointment of one or more staff members as their arbitrator, and their request shall be considered in making the assignment. Upon the submission of a request by one party for the appointment of a member of the agency staff as grievance arbitrator, the agency shall determine whether the other party to the collective bargaining agreement concurs in the appointment of a staff arbitrator. In the absence of concurrence, the agency shall notify the requesting party of the lack of concurrence and shall close the case if concurrence is not provided within a reasonable time.