Animating a Parrallax Loop Scroller?

So I managed to get this tutorial to work for me thus far:

But I cannot get another background to scroll slower in the background (both sides or just one side) to achieve the effect of a looping parallax.
Does anyone have any clues to achieving this?

Here is what I’ve done so far.

Made some terrain (plain, proportional editing to random, jerked up a few vertexes on the vertical).
Made an array of it.
Copied it and had it mirrored (so it’s also behind the camera / ship / car).
Then animated a camera on the x/y axis about 120 frames and parented (made a child) of the initial piece of terrain.
That gets it scrolling nicely, looping nicely, but still scratching my head to pull off some looping parallax.

Any ideas?