A reverse shadow (or what i call a Fake shadow) is going to be set with something like these:
Offset : 2
Such a shadow will show up on the top left side (while normal shadows will go on the bottom right) and will be quite faint (the exact settings will depend on the size of the project and the colors). Give it a try and see how it makes a more realistic separation between the layers. I might do a blog post tutorial about that in the future.