This glitch was found by MetalBlitzer425 and TheGreninja47, this glitch lets you play 2-8 characters at once, it works with the Gamecube Controller, Wii Mote, Wii U Gamepad, Classic Controller and the Wii U Pro Controller.
How to perform
1. Set P1's box to None
2. Disconnect the Controller and wait for the hand to disappear.
3. Connect the Amiibo into the game.
4. Go to Rules and set to whatever you want to, then go back to the Character Selection Screen.
After that the player should be able to control both P1 and P2, if there's 8 Players, it's possible to "fuse" them by rolling to one side of the stage, resulting in certain situations such as attacks clanking instantly after inputs.