Wash. Rev. Code § 81.04.140
Whenever any order of the commission shall require joint action by two or more public service companies, such order shall specify that the same shall be made at their joint cost, and the companies affected shall have thirty days, or such further time, as the commission may prescribe, within which to agree upon the part or division of cost which each shall bear, and costs of operation and maintenance in the future, or the proportion of charges or revenue each shall receive from such joint service and the rules to govern future operations. If at the expiration of such time such companies shall fail to file with the commission a statement that an agreement has been made for the division or apportionment of such cost, the division of costs of operation and maintenance to be incurred in the future and the proportion of charges or revenue each shall receive from such joint service and the rules to govern future operations, the commission shall have authority, after further hearing, to enter a supplemental order fixing the proportion of such cost or expense to be borne by each company, and the manner in which the same shall be paid and secured.
RCW 81.04.140