My friend and I have run into the same problem for our mod. It doesn't seem like you can have AI grid on multiple levels within the same area. Such a sad limitation. Can't think of any areas where they do it in the main game, so I assume it's impossible. The best you can do is add a door or cave or something and have that teleport you to the other side of the bridge.
Hope someone knows a way to overcome this, but don't have much hope.
Isn't there an area in northwest cyseal (city) where there is a treasure box in an area bellow a bridge? I think you can walk under the bridge there.
Not totally sure.
