Bob mounts the guns to the back of a truck to stabilize them.
Bob aims with one, and uses the other to lay down inaccurate suppressing fire.
Bob is ambidextrous, allowing him to operate two guns at the same time with equal accuracy for both.
Plotted A Good Waste: The ability to pull off this trope hints: Bob is more than an ordinary human. It turns out that when he was younger, Bob was one of the sixty children who were experimented on to be the supreme Badass. Bob is, by the way, the only one of those who is currently living a normal life (that is, if you consider being the leader of a team of five assassins to be a normal life).
Played For Laughs: The Scrappy does it to show how foolish he is.
Played For Drama: The Dragon does it to show how dangerous he is.