Twin Guns is the first upgrade of Path 3 for the Monkey Sub in BTD6. It doubles attack speed of the dart attack from every 0.70s to every 0.35s. It improves attack speed of the Ballistic Missile attack by ~14% (0.875x attack cooldown), and improves the attack speed for the submerged attacks from Submerge and Support and above by ~17% (0.85x attack cooldown).
Twin Guns improves the Monkey Sub's efficiency at soloing the first few rounds. Because of the Monkey Sub's base 2 pierce, it can shoot quite a number of homing darts before bloons start to exit its range. This makes a 0-0-1 Sub a popular upgrade in CHIMPS early-games.
This upgrade is usually used as a stepping stone to the next upgrade, Airburst Darts.
Twin Guns is useful for early-game due to its relatively fast attack speed. In BTD6, this upgrade is far more useful due to each dart having 2 pierce each instead of 1, and combined with the homing pattern of Monkey Subs it makes the 0-0-1 Sub a powerful early-game tower.
With Barbed Darts, the multitudes of darts sent by the Sub may have an easier time to chase out bloons. However, buying Barbed Darts restricts the opportunity to get Advanced Intel, which is often the better crosspath.