Are there any other witnesses that back up his claim? Kellerman's claim is probably best described as a flurry of shots which he later said was TWO shots. Kellerman, like MANY other witnesses, is describing the last sounds he heard that sounded like gunfire.
A list of witnesses from Richard Charmin's WORDPRESS blog that said the last two sounds were BANG-BANG (or words like that).
https://richardcharnin.wordpress.com/2014/02/08/jfk-math-analysis-witness-testimony-of-time-interval-between-shots/1. Special Agent William Greer, the limousine driver, testified that ?the last two shots seemed to be just simultaneously, one behind the other.? [66].
2. Secret Service Roy Kellerman sat next to Greer and was intimately familiar with the sound of weapons. Kellerman testified: ? Let me give you an illustration ? You have heard the sound barrier, of a plane breaking the sound barrier, bang, bang? That is it. It was like a doublebang ? bang, bang.? [67]
3. Special Agent George Hickey (in reference to the second and third shots). ?At the moment he was almost sitting erect I heard two reports, which I thought were shots and that appeared to me completely different in sound than the first report and were in such rapid succession that there seemed to be practically no time element between them.? [80]
4. Special Agent Clint Hill told the Commission that the second noise he heard was different from the first shot ? ? like the sound of shooting a revolver into something hard? almost a double sound.? [69]
5. S.M. Holland carefully watched the motorcade from the railroad overpass. He heard four shots with the third and fourth sounding like a ?double shot.? He thought some of the shots came from behind the fence on the grassy knoll.
6. Deputy Sheriff Roger Craig: ?The first shot ? sort of like it reverberated ? well, it was quite a pause between there [the first and second shots] ? It could have been a little longer [than two or three seconds]? ? Between the second and third shots there was ?no more than two seconds. It was?they were real rapid.? [71]
6. Joe R. Molina, witness ?? Of course, the first shot was fired then there was an interval between the first and second, longer than the second and third.? [72]
7. DPD Seymour Weitzman. ?First one, then the second two seemed to be simultaneously.? [73]
8. Ladybird Johnson. ? ? suddenly there was a sharp loud report?a shot. It seemed to me to come from the right, above my shoulder, from a building. Then a moment and then two more shots in rapid succession.? [74]
9. Secret Service Special Agent Forrest V. Sorrels. ?There was to me about twice as much time between the first and second shots as there was between the second and third shots.? [75]
10. Congressman Ralph W. Yarborough. ?? by my estimate?to me there seemed to be a longer time between the first and second shots, a much shorter time between the second and third shots?
11. Dallas Mayor Earle Cabell. ?There was a longer pause between the first and second shots than there was between the second and third shots. They were in rather rapid succession.? [77]
12. Secret Service Special Agent Sam A. Kinney. ?I saw the President lean toward the left and appeared to have grabbed his chest with his right hand. There was a second of pause and then two more shots were heard ? ? [78]
13. Special Agent William A. McIntyre. ?The Presidential vehicle was approximately 200 feet from the underpass when the first shot was fired, followed in quick succession by two more.
14. Special Agent Warren W. Taylor. ?In the instant that my left foot touched the ground, I heard two more bangs and realized that they must be gun shots.? [81]
15. Linda Willis. ?Yes, I heard one. Then there was a little bit of time, and then there were two real fast bullets together. When the first one hit, well, the President turned from waving to the people, and he grabbed his throat, and he kind of slumped forward, and then I couldn?t tell where the second shot went.? [82]
16. Special Agent Rufus Youngblood. ?There seemed to be a longer span of time between the first and the second shot than there was between the second and third shot.? [83] ? ? from the beginning at the sound of the first shot to the second or third shot, happened with a few seconds.? [84]
17. Robert Jackson. ?I would say to me it seemed like three or four seconds between the first and the second, and between the second and third, well, I guess two seconds, they were very close together ? ? [85]
18. Arnold Rowland. ?The actual time between the reports I would say now, after having had time to consider the six seconds between the first and second report and two between the second and third.? [86]
19. Luke Mooney. ?? The second and third shot was pretty close together, but there was a short lapse there between the first and second shot.? [87]
20. Ms. Mitchell (Mary Ann Mitchell). ?? there were three?the second and third being closer together than the first and second ? ? [88]
21. Lee Bowers ?I heard three shots. One, then a slight pause, then two very close together ? also reverberation from the shots.? [89]
22. Jean Hill. ?There were three shots ? one right after the other, and a distinct pause, or just a moment?s pause, and I heard more ? ? And concerning the shots that followed the first three Ms. Hill said they were ?quicker ? more automatic.? [90]
I prefer to hear the witnesses describe what they saw and heard rather than read transcripts. At 1 min 45 seconds in the YT video, Lee Bowers (#21 in the above list) describes the last two shots as "almost on top of each other."