That's not really parallax considering you only have 1 plane of field moving, it's more of a dynamic background. The secondary background should be moving at about 1/4 the speed of the primary background, and the very back should be the only stagnant image in the cycle. That way the girl and first background would be scrolling at an independent speed, rather than simply independently scrolling. Hope that helps.
[link] - If you notice all the independent movement in this, I think my explanation might make more sense.
[link] - If you notice all the independent movement in this, I think my explanation might make more sense.