Musashi Sensei (Miyamoto Musashi), wrote in his book "Go Rin no Sho"
One hand must be empty, free.
As well as they have to hold, each one, a sword. (when fighting with two swords at once)
This is Niten. This is Kenjutsu Combat.
When we practice to exhaustion, we can glimpse the beginning of the path to invincibility.