When two or more drainage districts outlet into the same ditch, drain, or natural watercourse and the board determines that it is necessary to clean out, deepen, enlarge, extend, or straighten said ditch, drain, or natural watercourse in order to expeditiously carry off the combined waters of such districts, the board may proceed as provided in section 468.126. After said board has decided that such work should be done, it shall fix a date for hearing on its decision, and it shall give two weeks’ notice thereof by certified mail to the auditor of the county wherein the land to be assessed for such work is located, and said county auditor shall thereupon immediately notify by certified mail the board or boards of trustees of the districts having supervision thereof, as to said hearing on said contemplated work. In those instances where two or more districts involved are under the supervision of the same board, or joint board if the district is intercounty, the notice shall be given to all landowners affected as prescribed for in sections 468.14 through 468.18. Each district shall be assessed for the cost of such work in proportion to the benefits derived. Common outlet for the purpose of this section shall mean an outlet where two adjacent districts have an outlet common to both of said districts and which districts are also contiguous, one to the other.