Whenever the commissioners of highways and township boards of any 2 adjoining townships in either the same or different counties shall determine that a bridge on or across the township line over any stream or sink-hole, either crossing or running along the boundary line between such townships, should be constructed, rebuilt or repaired, such bridge shall be constructed, rebuilt or repaired and maintained at the joint expense of such townships. The commissioners of the townships interested, either of whom may give notice to the other of meetings of said commissioners for the purposes herein mentioned, shall jointly agree on the plans and specifications for such bridge, subject to the approval of an engineer assigned to the job by the state highway commissioner in case the structure shall cost more than 2,500 dollars, and upon the proportion of the cost thereof that each such township shall bear.