Hmm I cant picture the style that you are wanting to achieve, maybe if you drew a picture it would help.
But, lets say you wanted the far set of mountains to have the normal X parallax but you want to control when it moves up or down (so its not actually parallax for the Y), you would then have to manually move it up or down.
Simplest way to do this is add bullet behaviour and control the speed and angle of motion.