I had the exact same thing happen to me once using Golden Gun. The 2 guys were stacked exactly like they are in your video. When the GG shot passed between them it hit the guy in the back and killed them both. I didnt have keyhole or combustion on either. The game just gave me credit for both because it was too close maybe. I think the game just has a problem when they are in close proximity like that at that angle. Now that I think about I have got a few double kills using a shotgun in this same scenario. One shot giving out 2 kills.