Obviously, we know that the whole "jumping in the air" part is probably a slight exaggeration on Hollywood's part, but the rest is simple arithmetic: If one gun can kill a lot of enemies, two guns should kill twice as many, right?
Why This Sucks:
Here's a quick challenge: Find us a photo of a police officer, soldier, or any other professional doing this on the job (and not just posing for a photo, either). You won't, because using two guns makes both of them useless.
"Count the number of guns m**********r ..."
And don't try playing the "it'd be possible if you trained hard enough" card -- the problem is human biology. As you may have noticed, the binocular-like way in which our eyeballs work means we can't see independently with each one, a skill we'd need to aim two guns at the same time at completely different angles, all whilst dodging from cover-to-cover and experiencing massive amounts of recoil that causes the gun sights to bounce all over the place. Enjoy this clip of the certifiably insane hair-beasts over at Mythbusters trying to aim at two targets with two guns and hitting none: