The attachment decreases the time needed for every other reload action by using two magazines taped together. Every odd reload (e.g. first, third, fifth, and so forth) is faster. The following reload will be the default, original reload. The odd reload depicts the player removing the spent magazine, and flipping it around (either on screen or off screen), and inserting the second, fresh magazine.
The attachment also add 50% more ammo when the player spawns (i.e. FAMAS starts with 120 rounds normally, with dual mags the player spawn with 180 rounds, a 60 round increase.)
Since it speeds up reload times and provides more ammo, it reduces the need for Sleight of Hand and Scavenger. Because of this, it is a strong attachment choice as it opens up tier 1 and 2 slot to other perks such as Flak Jacket and Hardened to maximize killing power.