(1) If a dual member becomes disabled, the member's service in all systems may be combined for the sole purpose of determining the member's eligibility to receive a disability retirement allowance from the member's current system.

(2) The member's current system shall use its own criteria to:

(a) Determine the member's eligibility for a disability retirement allowance; and

(b) Calculate the disability retirement allowance based on service actually established in the current system. The member shall be allowed to substitute the member's base salary from any system as the compensation used in calculating the allowance.

(3) Subsections (1) and (2) of this section shall not apply to the member's prior system.

(4) A dual member who is eligible to receive a disability retirement under the current system may elect to receive a service retirement from all prior systems and to receive service retirement allowances calculated as provided in this section. Each system shall calculate the service retirement allowance using its own criteria except that the member shall be allowed to substitute the member's base salary from any system as the compensation used in calculating the service retirement allowance.

(5) The service retirement allowances from a system which, but for this section, would not be allowed to be paid at this date based on the dual member's age, may be received immediately or deferred to a later date. The allowances shall be actuarially adjusted from the earliest age upon which the combined service would have made such dual member eligible in that system.

(6) This section shall not apply to any disability benefit under:

(a) RCW 41.40.220; or

(b) The Washington state patrol retirement system established under chapter 43.43 RCW.