I thought that also until I found about a half dozen twitter feeds and youtube/live leak videos.
The distinct "two guns firing" is actually one gun, with two different sounds: 1) The initial "BANG" when a bullet leaves the barrel, and 2) about 30 feet out front, the bullet goes supersonic, and you hear a "crack/snap".
There are two (2) videos I'll try to post that makes it clear that it is one, and only one gun firing making both the boom and then the crack.
There is also an air foil/vortex behind the bullet that makes the "whizzz/buzz" sound as it goes by.
Then of course there is the ominous "Thwack" sound that is terrible to hear: when a high velocity bullet hits flesh and creates the hydrostatic tissue damage.
Thanks for the info. Is there a grassy knoll in the area?