I have a question! *shockhorror*

I was pondering today how one would go about creating bridges in a game where you can go over the top of them (and not fall off, even if you tried) and go under them as well. Like so:

If the players walked under and over, and happened to cross paths, they wouldn't bump either!

I have no idea how this would work. Please enlighten me! :)
In response to Artemio
Artemio wrote:

Garthor, you really are quality. :p

Thanks, Artemio!