There is a fourth distance you can define called the spread offset, which moves the shadow in or out on all 4 sides. So if you set that to the negative of the blur distance, this will shift the shadow inwards by the same distance as the blur extends the shadow outwards effectively hiding it. Of course this will also shift the shadow inwards on the side you do want it to appear so you’ll need to increase the offset by the blur distance to undo that. i.e.
box-shadow: (horizontal + blur) 0px (blur) (-blur) color;
So in your example:
box-shadow: -8px 0px 5px -5px rgba(0,0,0,.8), 8px 0px 5px -5px rgba(0,0,0,.8);