You mean something like this?
http://doptrix.com/C2/EndlessScroller/ Endless scroller
I wasn't very accurate with the green line, you should make sure that it enters and exits each frame seamlessly at the exact same point.
http://doptrix.com/C2/EndlessScroller/EndlessScroller.capx CAPX
what if instead of a background I wanted to make a foreground with enemies and platforms