I suppose there is a bug with overlapping:
object A is on layer A, whose Parallax parametre is 100, 100
object B is on layer B, whose Parallax parametre is 0,0
When scrolling , object B moves with the scroll, and it can not any more "feel" the overlapping, which worked before the scrolling.
There is an offset
So I suppose the overlapping should calcul with position relatif but not absolute.
<img src="smileys/smiley17.gif" border="0" align="middle" />