Mo. Rev. Stat. § 67.685

Any two or more counties levying a tourism sales tax throughout or in any area of the county under the provisions of sections 67.671 to 67.685 may act cooperatively in carrying on activities for the advertising and promotion of tourism activities within their respective areas as authorized by the provisions of chapter 70; and, in such event, at the end of the first full year in which the tax is collected, the proceeds from the tax in all cooperating counties shall be totaled. If in any one cooperating county at least forty percent of the total amount was collected, the governing body of that county shall appoint an additional member to the board; if at least sixty percent of the total was collected in one county, the governing body of that county shall appoint two additional members to the board, with all of such additional appointments being made from the general public of the county from which the appointment is made.