you cant do multiple leval pathing in warcraft. you need to have triggers that remove the bridge when a unit goes to walk under it, and replace it with the same model but with no pathing, if you do it with more than one unit however you encounter problems such as one unit being on the bridge, then when another unit goes under it, the bridge will be replaed, and the unit on top will drop to groind leval. so you can only do this effectivly in RPG's unfortunately | 
