If a bridge or proposed bridge is on the dividing line between two counties which is also the dividing line between one county and a municipality in the other county and the municipality has authority to build or rebuild the bridge or to join with any county therein, the county may join with said municipality in the other county in building or rebuilding the bridge. The cost of the bridge shall be paid in the relevant proportions as shall be agreed upon by the county and municipality that joined.
Pennsylvania Legal Code