Boshy, Double Shot glitch

I wanna be the Boshy, Double Shot glitch:

If you press [Shoot] and another button (e.g. [Down]) at the same time, you fire two bullets at once instead of one. You can either hit both inputs at once, or simply rebind the [Down] button to the same key as the [Shoot] button.
Pressing CTRL + Y calls up the rebind-buttons menu.

Certain bosses only take 1 damage from the double bullets, and are thus faster to defeat not using this glitch, because you can only have a maximum of 5 bullets on the screen at once. Said bosses are:
- Hello Kitty (Boss 1)
- King Dedede (World 2 miniboss)
- Skeleton King (Boss 6)
- Shang Tsung (Boss 8)
- Ganon* (Boss 9)

* For Ganon it doesn't matter, since you are near enough to him so you are not limited by the amounts of bullets on the screen.