I don't know much about expressions, but what I did was to put the
back side of the page/cover 1 pixel deeper in Z space. Then I parented the back side to the front side. Now when I turn the front page, the back page moves with it.
While it doesn't show the back side of the pages very much, I used this technique to create this video...
Mark Fogarty
Media Missionary
Muddy River Media
www.MuddyRiverMedia.org