There are many enemies

Musashi is held to be the inventor of the Two Sword style. His school is sometimes called "Nito Ryu" (two sword school) and sometimes "Niten Ryu" (two heavens school). He writes that the use of two swords is for when there are many enemies, but people practise a style of fencing with a sword in each hand to give practical advantage in fencing. Musashi used the words "two swords" when meaning to use all one's resources in combat. He never used two swords when up against a skilled swordsman.