Wash. Rev. Code § 88.32.190
In every case of such joint action, the preliminary procedure of RCW 88.32.010 having been first had in each county severally, the board of county commissioners of the several counties proposing to join shall unite in such an application as is prescribed in RCW 88.32.020, and the application shall be made to any person, who, for the time being, shall be a judge of the United States district court in any district in which such counties, or any of them, may lie, and the list mentioned in RCW 88.32.020 shall be made in as many counterparts as there are counties so joining, and one counterpart shall be filed with the board of county commissioners of each county, and if the person who is such United States judge shall decline or be unable to act, then, the board of such counties shall meet in joint session, at the county seat of such one of the counties as shall be agreed upon and shall organize as a joint board by appointing a chair and clerk, and by resolution in which a majority of all the commissioners present, and at least one commissioner from each county, shall concur, name the eleven persons for the commission, which eleven in such case shall be citizens of the counties concerned, and as nearly as may be the same number from each county. A counterpart of such resolution shall be recorded in the minutes of the proceedings of the board of each county. The commission shall make as many assessment rolls as there are counties joining and one counterpart roll shall be certified by such chair and clerk of the joint board, and by such clerk filed with the board of each of such counties.
RCW 88.32.190