The closest you can get is to make two regions near the tunnel entrances and move units from one side to another.
You could plateu lowered ground and build a bridge over? Like a tunnel.
I already tried that, and it appears the bridge is treated as a doodad from the ground, so you can't pass under. I also tried increasing the bridge cliff height and removing pathing, but all that does is that units transition from tunnel to bridge like a big jump.