Wash. Rev. Code § 35.13.320
The councils of any two cities that will be in a situation described in RCW 35.13.310(1) as the result of a proposed annexation by one of the cities may enter into an agreement to adjust those portions of the annexation proposal and the boundaries of the city that is not proposing the annexation. Such an agreement shall not be effective unless the annexation is made.
The annexation proposal shall proceed if such an agreement were not made, but any resulting boundaries between the two cities that meet the descriptions of RCW 35.13.310(1) shall be adjusted by agreement between the two cities within one hundred eighty days of the effective date of the annexation, or the county legislative authority of the county within which the right-of-way is located shall adjust the boundaries within a sixty-day period immediately following the one hundred eightieth day.
An agreement or adjustment made by a county under this section shall include only boundary line adjustments between the two cities that are necessary to eliminate the situation described in RCW 35.13.310(1).
A boundary line adjustment under this section is not subject to potential review by a boundary review board.
RCW 35.13.320