To answer the question of why there would be two BL's, let's first consider the nature of JFK's fatal head wound. To do this I will be referring to an article written by Dr. Gary Aguilar titled:
JOHN F. KENNEDY'S FATAL WOUNDS:
THE WITNESSES AND THE INTERPRETATIONS
FROM 1963 TO THE PRESENT
found at the address: assassinationweb.com/ag6.htm
There are three main things I want to emphasize from this article (focusing on the Parkland witnesses):
1) vast majority of Parkland witnesses recalled seeing a large gaping wound on the right, rear side of JFK's head:
"...twenty of twenty-two Parkland witnesses...first, unrehearsed, and specific recollection of the head wound, either in writing, verbally, or both, placed a major skull defect unambiguously posteriorly on the right."
2) vast majority of Parkland witnesses did NOT recall seeing an entry wound somewhere on the front of the head which could account for the large, gaping wound on the back of the head. Those few that did recall seeing such a wound have been dismissed, according to Dr. Aguilar:
"It must be mentioned that there were those at Parkland who described a left temporal entry: Robert McClelland, MD, Marion Jenkins, MD (WC--V6:48) and Father Oscar Huber all mentioned such a wound.(BE:46, 331) This location for a wound has been abandoned by all: the Warren Commission loyalists by arguing that Oswald was firing from the school book depository, while some Warren Commission critics rejected it for conflicting with a grassy knoll origin and left-rear head thrust."
3) Four separate Parkland doctors used terms such as 'tangential', 'tangentially', 'avulsive', and 'avulsion' to describe the head wound on right, rear side:
Professor and Director of Neurological Surgery at Parkland, Dr. Kemp Clark: "The head
wound…could have been a
tangential wound, as it was simply a large, gaping loss of
tissue."
Anesthesiologist at Parkland, Dr. Gene Aiken: "I assume the right occipitalparietal
region was the exit, so to speak, that he had probably been hit on the other side of the
head, or at least
tangentially in the back of the head..."
Dr. Charles James Carrico: "There seemed to be a 4 to 5 cm.
area of avulsion of the scalp
and the skull was fragmented and bleeding cerebral and cerebellar tissue."
Dr. Malcolm Perry: "...I noted a large
avulsive wound of the right parietal occipital area,
in which both scalp and portions of skull were absent, and there was severe laceration of
underlying brain tissue..."
In medical terms an avulsion is an injury caused by the tearing away of a body part.
Taking these facts together, we come to the following scenario: the assassin's bullet tangentially impacted the right, rear side of JFK's head, which tore out (avulsed) a portion of his skull in this area.
Now, who was behind and to the left of JFK and had a clear line of sight to the right, rear area of his head?
I know some will laugh at me, but I think, in all good faith, we should seriously consider the following scenario:
BL_1, the BL present when JFK was killed, was really an assassin in drag - dressed up as a frumpy old woman. BL_1 killed JFK.
BL_2, an actual 'woman with a camera', took the place of BL_1 soon after JFK was killed. This serves to reinforce the initial impression we get from looking at BL_1, and serves to throw us off the scent. We say to ourselves something like: 'The babushka lady was just a woman with a camera, and while it would be interesting to know who she was and what she filmed, it's not really crucial to find these things out' And then we keep scrutinizing the 'grassy knoll' for something - anything.
This tactic allows the conspirators to place a shooter, hiding in plain sight, in close proximity to JFK AND behind him. Being behind JFK is important, because the plan is to pin the blame on Oswald, who was supposed to be shooting at JFK from behind.
Now, obviously, an avulsion on the right, rear side of JFK's head was not the desired outcome. But look at the Muchmore frame above: JFK is hunched over and Jackie is leaning toward him. This give BL_1 a smaller target to work with. He wants to hit JFK in the back of the head, but he absolutely must avoid hitting Jackie - or the 'lone gunman' story is toast. So BL_1 was forced to aim further off center of JFK's head, which resulted in the bullet impacting tangentially.