The Japanese military art founded by Miyamoto Musashi and associated with two-sword fighting is Niten Ichi-ryū.
Musashi developed the school during Japan’s early Edo period and described its principles in The Book of Five Rings. The name is often translated as “Two Heavens as One,” and the tradition teaches the simultaneous use of a long sword and a short sword. This method is commonly called nitō kenjutsu, meaning two-sword swordsmanship.
Niten Ichi-ryū is not simply a rule that every samurai always fought with two blades. Japanese warriors used different weapons and tactics according to circumstances, and Musashi’s school emphasized timing, distance, posture, initiative, and practical combat judgment. Musashi is also associated with a famous duel against Sasaki Kojirō, although later accounts contain legendary elements. Several modern branches preserve traditions connected with the school, so its exact techniques and lineages can vary between organizations.