Wash. Rev. Code § 88.32.200
For purposes of a board of equalization, said boards shall from time to time meet as a joint board as aforesaid, and have a chair and clerk as aforesaid, and for all purposes under RCW 88.32.070 and 88.32.080, in case of counties joining, the word board wherever occurring in said sections shall be interpreted to mean such joint board, and the word clerk shall be deemed to mean the clerk of such joint board, and the posting of notices shall be in at least ten public places in each county, and the publication of the same shall be in a newspaper of each county, and the objections mentioned in RCW 88.32.080 shall be filed with the clerk of the joint board, who shall cause a copy thereof, certified by him or her to be filed with the clerk of the board of county commissioners of the county where the real estate of the party objecting is situated.
RCW 88.32.200