R.I. Gen. Laws § 36-9-2.1: Membership in other retirement systems.

(a) An employee who is a member of the retirement system may be a member of the retirement system of any union provided the union retirement plan is covered by the Employee Retirement Income Security Act of 1974 (ERISA) [29 U.S.C. § 1001 et seq.] and is tax qualified pursuant to the internal revenue service, and remain eligible to participate and receive benefits as provided in chapters 8 through 10 of this title.

(b) Participants in a union retirement system shall not receive a greater wage or benefit increase then that granted other state employees covered by a collective bargaining clause.