Mo. Rev. Stat. § 67.458
The governing bodies of two or more adjoining counties may, pursuant to section 70.220, contract to improve a road or street located within such adjoining counties. In addition, the governing bodies of two or more adjoining counties may create a neighborhood improvement district for the purpose of improving a road or street located within such adjoining counties. Except as otherwise provided in this section, all provisions of sections 67.453 to 67.475 shall apply to such a district and all powers included within sections 67.453 to 67.475 shall be available to the governing bodies of the district; however, any decision required of the governing bodies under sections 67.453 to 67.475 must be made in a unanimous manner by all governing bodies of the counties in the district. In forming such a district, the governing body of each county shall separately comply with the provisions of either subsection 2 or 3 of section 67.457, and all proposed portions of the district must be joined as part of the district or the district shall not be formed. The separate fund or account required by section 67.473 shall be a fund or account maintained in the county treasury of the county containing the largest percentage of the assessed valuation of the district; however, the governing body of each county within the district shall be required to approve expenditures from the fund in accordance with section 67.473.
§ 67.458, RSMo