Start with a graphics program that has transparent layers, free transform and image flip.
Free on-line one at Pixlr. (Note: Text tool doesn't work in browser private-mode.)
Jerry, I've tried some of this with a Power Point program... However, the quality of the photos are different as well as the angle...
Again, a good up close HD image of the National Archives windshield would determine if these are the very same bullet cracks.
Also, an up close examination would allow a glass expert to determine if the bullet crack came from the front or the back of the windshield.
If it came from the front, two questions arise: 1) Who made the shot that cracked the windshield 2) Was the shot intended for SA Bill Greer, who might have known too much about the assassination details (the same reason it could be claimed Oswald was killed... to shut him up)