Double Gun is the 3rd upgrade of Path 3 for the Engineer Monkey. It allows the Engineer to attack with two nailguns instead of one, allowing the Engineer to attack twice as fast alternating with each of its guns. Double Gun inherits the properties of the preceding Pin upgrade. It only affects the Engineer's nailgun and not its sentries.
It costs $425 on Easy, $500 on Medium, $540 on Hard, and $600 on Impoppable.
Generally speaking, a 0-2-3 Double Gun is better than a 2-0-3 Double Gun. This is because the extra range from Larger Service Area is highly beneficial for reaching more bloons at once with the Engineer's fast guns. Additionally, the Deconstruction upgrade adds extra damage to blimps and fortified, a highly useful upgrade at chipping down those bloon types.
This upgrade is a helpful early-game tower for general stalling of bloons for a rather low price.
The concept of the Double Gun upgrade is likely a reference to the Engineer from Team Fortress 2, specifically from the Meet the Engineer trailer where Engineer tells the viewers that he solves practical problems regarding self-protection with "more gun": "The answer? Use a gun. And if that don't work, use more gun."