Two patterns muddy the picture. What if one barrel is high left and the second high right? When combined you think ok its high. If you want to know where a gun shoots it takes a lot of patterns. One shot does not do it. Five might and ten should. Then you have to be shooting it in a normal way. Shooting off sand bags is not going to tell you much unless you intend to shoot off sand bags all the time. I had a dove gun which shot differently in the field than it did on paper. Almost all my shots in the field were from a sitting position while my patterning was done standing up. Different mounting and different poi. Took my awhile to grasp that one.