One Actor, 2 Levels

Hello all!
I'm working on a platform game  today and encountered a problem that i dont know how to face.
Take this pic:

There are 2 walking levels. One is front, the second one is supposed to be on the back, and the character can swap from one to another in particular places, like bridges. They arent on same Z! (X is horizontal, Y vertical).
In poor words it should be like a simulated 3d, a very good example of it in a game is Tomba for psx.
My problem is that working with tiles i cant have 2 groups of tiles so the actor collide with a group in the front and with the other group in the back. Right now if the actor hit a "back" tile it collides.
Someone knows or can think of a solution without using actors for the back level platforms?

(im sorry if its a bit confused xD)

Terrains?

Terrains are just collision shapes, i cant activate-deactivate them or set different groups, they counts as tiles.. need something else.

Without using actors, you would have to have some kind of trigger to tell the walking actor that there was something solid there and to act different; this would probably have to be a region (I doubt you want to hard-code dimensions and coordinates of the place). When the actor enters, you would have to push them up until they reached the top of the region.

In short, actors would be better.

I see, thank you.
I'm thinking to make no tile's collisions and rely completly on invisible actors that switches from solid to sensor with the level i'm on.