How do you unlock master hand in ssbm

You need to go to multi player and set your melee settings.
put 1 controller in slot 3 and one in slot 1
You must have a hmn or cpu normal character
select pl1 character
regester pl3 as human but don't select a character
move pl1 controll to back
move pl3 selection to name entry
open name entry
hover over a name
press b and a on both controllers
sometimes it won't work.

This also works for controlling the same color character ex-Green mario is all 4 players

You can have cpu master hands and more hmn master hands (all hmn master hands are controlled in socket 3)

Youtube has videos on this.