In any county where there is not a sufficient amount of road and bridge work to keep the county engineer employed throughout the year, the board of county commissioners of any such county, with the approval of the secretary of transportation, may unite with the board of county commissioners of an adjoining county or counties and form a county engineer district and employ a county engineer for the two or more counties included in such district. The boards of county commissioners of any such county engineer district, with the approval of the secretary of transportation, may reduce or increase the size of the district or dissolve the district as the conditions may warrant or demand, but no district so formed shall be altered or dissolved in less than one year from the date of such formation.