Joint district sewers may be constructed by the county commission whenever the county commission deems it necessary that a sewer shall be constructed in any part of a sewer district extending into two or more districts. In such case, it may, by resolution, unite contiguous sewer districts into a joint sewer district and cause a sewer to be constructed therein in like manner and in all respects as is provided in the case of district sewers. The cost of constructing said joint district sewer, less any amount procured from federal or other governmental source, shall be assessed and paid in special tax bills against the property included in the joint sewer district, the same as provided in the case of district sewers. The action of the county commission creating the joint sewer district shall be conclusive as to the necessity therefor, and no special tax bills shall be held invalid or be affected on account of the included drainage area thereof, or the size, character or purpose of such sewer; provided, that no sewer district shall be included in such joint district which is not included in the natural drainage area in the valley or watercourse in which the joint district sewer is proposed to be constructed, or which may be conveniently served in said area.