Wash. Rev. Code § 35.10.490
A joint resolution or the petitions may prescribe the name of the proposed consolidated city or may provide that a ballot proposition to determine the name of the proposed consolidated city be submitted to the voters of the cities proposed to be consolidated. If two alternative names are submitted, the name receiving the simple majority vote of the voters voting on the question shall become the name of the consolidated city. If the name for the proposed consolidated city is not prescribed by the joint resolution or petition, or a proposition on the name is not submitted to the voters of the cities proposed to be consolidated, then the newly consolidated city shall be known as the city of . . . . . . (listing the names of the cities that were consolidated in alphabetical order). The legislative body of the newly consolidated city may present another name or two names for the newly consolidated city to the city voters for their approval or rejection at the next municipal general election held after the effective date of the consolidation. If only one alternative name is submitted, this alternative name shall become the name of the consolidated city if approved by a simple majority vote of the voters voting on the question. If two alternative names are submitted, the name receiving the simple majority vote of the voters voting on the question shall become the name of the consolidated city.
RCW 35.10.490