That conclusion doesn't follow from the stated premises. The most basic missing premise is that Oswald was positioned (inside the book depository) behind the motorcade. See https://upload.wikimedia.org/wikipedia/commons/5/58/DealeyPl... Of course, there are many other necessary premises wherein lie the interminable debate. But if you assume (as it seems the doctor did) that at least one shot (possibly a miss) came from Oswald behind the president, who was facing forward; and one shot came from ahead of the president exiting the back of his head, the scenario the doctor believed the evidence demanded; then you must conclude at least two shooters.