Double Shot is the third upgrade of Path 1 for the Ninja Monkey. This upgrade allows the Ninja Monkey to throw 2 shurikens at once, allowing a maximum poppage of 8 bloons. This is good at popping Blue Bloons, as it pops both layers, as well as spaced bloons.
Double Shot has always been very reliable against spaced bloons, particularly against Spaced Pinks and Spaced Blacks. It used to be too expensive to buy, but it now has been balanced to a much more fair price.
Purchasing Double Shot no longer decreases the firing speed of the Ninja Monkey (unlike BTDB flash).
In the flash version of Bloons TD Battles, Double Shot slightly decreases the fire rate of the Ninja Monkey, as if it lost the Ninja Discipline ability (without losing the range). This does not happen in the mobile version of the game.