Double? How about triple? Check out (one of my)
Multi Level Layout(s )

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To answer your question:
It is possible, but not with two layers of actual Trainz ground; you will have to "fake" it by using other assets. In my case I made the different levels in Blender and added them as assets to the game, but I have seen others do it with splines of wood or grass or something. You end up needing to think "out side the box" and be creative with a either self-made assets or (ab-)use assets initially not made for it.