Whenever a bridge, ferry, or causeway is necessary on the line between two counties, the same must be built at the joint expense of such counties, in proportion to the amount of taxable property in each. When the cost of building any county line bridges is so great that it cannot be paid by the counties which such stream divides, which fact may be determined by the county commission of either county, such counties, or either of them, may make such contracts and incur such debt as may be necessary, within constitutional limits, to procure for the use of such counties, or either of them, or the inhabitants thereof, the free use of a thoroughfare for vehicles and foot passengers over or across any bridge that may be built by any person, firm, or corporation over any stream forming the dividing line between such counties; and to pay any debt that may be thereby incurred, any county may issue its bonds when such issue is authorized by vote as required by law. A county line bridge or ferry, in the meaning of this section, shall be construed to be a bridge or ferry across a stream or slough between two counties whether the actual dividing line between said counties runs on either bank or in the center of said stream or slough. If a toll bridge, ferry, or causeway is in operation on any such line, the same may be purchased by either county alone or at the joint expense of the counties connected thereby, in proportion to the taxable property in each.