Wash. Rev. Code § 86.13.050
When such a contract shall have been entered into and occasion shall arise for the joint action of the two boards of county commissioners whether such joint action is provided for in this chapter or otherwise desired upon any matter having relation to such contract or the prosecution of such improvement, such joint action may be secured by a notice calling a joint meeting signed by two county commissioners, designating the time and place in either county of such meeting, served by one of the two county auditors upon the remaining county commissioners at least seven days (exclusive of the date of service or mailing) prior to the time so designated. If the notice is signed by two county commissioners of the same county the place of meeting shall be at some place in the other county designated in the notice. Such service may be personal or by mail addressed to the member in care of the county auditor of his or her county. The six county commissioners may constitute a legal meeting without notice by being present together for that purpose. The auditor's certificate of such personal service or mailing, attached to a copy of the notice, shall be made a part of the records of the meeting and be competent proof of the fact. Except in the case hereinafter provided for, the presence of four of the county commissioners shall be necessary to constitute a legal meeting. Each meeting shall be presided over by one of those present selected by vote. The county auditor of the county wherein the meeting is held shall be secretary of the meeting, and shall make duplicate record of its proceedings, one of which, with his or her certificate thereon, shall be forwarded to the county auditor of the other county, and such record shall be a part of the record of the board of county commissioners of each county. A majority vote of those present at any legal meeting shall be determinative upon any question properly considered at the meeting, and shall be binding upon each county as if enacted or adopted by its own board of county commissioners separately, but no joint meeting whatsoever shall in any manner continue, extend, change, alter, modify, or abrogate the contract when made or any of the terms and conditions contained therein. Each county commissioner shall be paid out of said fund in his or her own county all disbursements made by him or her for traveling and other expenses incurred in attending any joint meeting or in any way connected with the prosecution of the improvement. Any legal meeting shall have power to adjourn to another time and place. An adjourned meeting shall have all the powers of the meeting of which it is an adjournment, but shall have no power after the end of the thirtieth day following the date of the original meeting of which it is an adjournment. If the three county commissioners of either county shall fail to attend any two meetings consecutively called, the notice for the next succeeding meeting may be also served upon the special commissioner hereinafter provided for, and if he or she and three county commissioners attend pursuant to such notice the four shall constitute a legal meeting, but if he or she does not so attend and three county commissioners do attend, the same shall constitute a legal meeting: PROVIDED, All notices calling a joint meeting shall specify distinctly and separately each question to be considered at said meeting; and it shall be unlawful to consider any question at such meeting or at any adjourned meeting thereof except those which have been distinctly and separately specified, except in cases where all six county commissioners are present or five county commissioners present are unanimous on the question, and in any action which may be taken on any question other than those specified in the notice shall be void and shall not be binding on either county, except in cases where all six county commissioners are present or the action was by unanimous vote of five county commissioners present at such meeting.
RCW 86.13.050