Do you mean you want to have them fall as a mirror image of the forward shatter?
If you just want to rotate the back side, it is a simple matter of adding 180 to the y-rotation (of the front camera).
If you want a mirror image, you’ll have to put something like 180 minus y-rotation (of front camera), and then you have to flip the backside (or frontside) image. Gets more complex if you have moved the camera off center, so better keep it in the center if possible.
…if I understand what you are trying to do correctly.