Wash. Rev. Code § 85.08.570
When a drainage, diking, or sewerage system is proposed which will require a location, or the assessment of lands, in more than one county, application therefor shall be made to the board of county commissioners in each of said counties, and the county engineers shall make preliminary reports for their respective counties. The lines of such proposed improvement shall be examined by the county engineers of the counties wherein said improvements will lie, jointly. The hearings in regard to such improvements, provided for by RCW * 85.08.150, and 85.08.400 through 85.08.430 shall be had by the boards of county commissioners of the two counties in joint sessions, and all other matters required to be done by the county commissioners in regard to such improvement and the improvement district shall be had and done by the boards of county commissioners of the counties wherein such system of improvements shall lie, either in joint session at such place as the said board shall order, or by concurrent order entered into by the said boards at their respective offices. Notice of the hearings shall be given by the auditors of both counties jointly by publication in the official paper of each of said counties. The county engineer of the county wherein the greatest length of drainage, diking, or sewerage system will lie, shall have charge of the engineering work and be ex officio a member of the boards in this chapter provided for. The schedule of apportionment shall be prepared in separate parts for the land in the respective counties; and that part of said roll containing the assessments upon the lands in each respective county shall be transmitted to the treasurer thereof, and the treasurer of said county shall give notice of said assessments as provided in RCW 85.08.400 through 85.08.430, and shall collect the assessments therein contained and shall also extend and collect the annual maintenance levies of said district upon the lands of said district lying in his or her county. The auditor of the county in which the greater length of the drainage, diking, or sewerage system shall lie shall act as clerk of the joint session of the boards of county commissioners, and shall issue the warrants of the improvement district, and shall attest the signatures of the two boards of county commissioners on the bonds. He or she shall furnish to the auditor of the other county duplicate copies of the records of proceedings of such joint sessions. Duplicate records of all proceedings had and papers filed in connection with such improvements shall be kept, one with the auditor of each county. Protests or other papers filed with the auditor who is not clerk of the joint sessions shall be forwarded forthwith by him or her to the auditor who acts as clerk of such joint sessions. The treasurer of said county shall register and certify and pay the warrants and the bonds, and shall have charge of the funds of the district; and to him or her, the treasurer of the county in which the lesser portion of such system of improvements lie, shall remit semiannually, in time for the semiannual warrant and bond calls, all such collections made in such other county. A drainage, diking, or sewerage improvement district lying in more than one county shall be designated "joint drainage (or diking) or sewerage improvement district No. . . . . of . . . . . . and . . . . . . counties." All proceedings in regard to joint drainage, diking improvement districts, which have heretofore been had and done substantially in accordance with the amendatory provisions of this chapter are hereby approved and declared to be valid.
RCW 85.08.570
*Reviser's note: RCW 85.08.150 was repealed by 1985 c 396 s 87. See RCW 85.38.040, 85.38.050.