Two individuals may share one employee position in any State agency. “State agency” means all departments, officers, commissions, boards, institutions and bodies politic and corporate of the State, including the offices of Clerk of the Supreme Court and Clerks of the Appellate Courts, the several courts of the State and the legislature, its committees or commissions. “Shared position” means an employee position in any State agency in which 2 individuals share the salary and employee benefits. Participation in a shared position must be voluntary on the part of the employee. Where employees in a state agency form a recognized collective bargaining unit or are subject to an existing collective bargaining agreement, shared positions shall be subject to negotiation. Seniority must be accorded to employees in shared positions under the same terms as all other employees.