I remember reading about these. As I recall it seemed rather complicated a first but I think there is a youtube video that clarified things.
the two reticals move separate from each other. So, fire into the target, then while looking through the scope at the target you just shot, keep the cross hairs on the center of the target (or your original point of aim), but move the circle to the bullet hole. You should now have the circle corresponding with the bullet hole while the cross hairs are on the center of the target at the same time. then move the cross hairs to where the circles are.
I have employed the same concept with a regular scope, its just much more difficult because the rifle can't move at all once your start moving the cross hairs. With the Shepard scope, the cross hairs remain your reference mark. I was under the impression you don't need shims like with the old soviet scopes, because I believe the field of view moves with the cross hairs. maybe I was wrong on that?