I have found a pretty cheap way to pass some of these levels with minimal resources. You can simply running a steel cable underneath the wooden bridge and connect a few nodes with vertical pieces of wood. It will not be stable at all, but as long as both vehicles can get across it doesn't matter, right? I have taken screenshots of each of my set-ups and included them below for your use. These set-ups also work for the trucks. To save time, only run the trucks over these bridges to unlock this stackable achievement.