I'm pretty sure Coolty44's option is the only 'sensible' way, it's like invisible platforms, if you place one high up for a model to stand on, a unit cannot exist underneath it. I don't think wc3 supports multiple height layers/on top of one another.
I suppose you could make it 'look' like they are walking under it, via height/collision triggers when they enter/leave regions along either side of the bridge (adjacent to the non-pathable pink bits).