If you change X coordinate for both objects to something like 150, you'll see that they are aligned at the moment of creation.
But since you are creating them at x=1250, when character moves in their direction, it takes twice the distance to get to the "back" sprite, because it's on 50% parallax layer.
If you want these objects to always be aligned, you need to adjust "back" position on every tick. Or simply place it on layer with 100% parallax.