A Wakizashi, meaning side inserted, is a Japanese short sword used as a backup weapon and for close quarters fighting. Wakizashi are designed to be used one handed and feature blades between 12 and 24 inches. Longer wakizashi are called o-wakizashi.
Dual wielding both a katana and wakizashi was rarely formally taught, but was used by some notable swordsman such as Musashi. There were several schools in Japan that specialized in wakizashi use, and in close quarters combat these tactics were put to deadly use. The Japanese film When the Last Sword is Drawn focuses on a samurai who specializes in close quarters combat with a wakizasi and has an amazing fight scene with katana vs wakizashi.
Ronin Katana Sword Company offers three models wakizashi of wakizashi for sale in the dojo pro line. They can be pared with matching katana to form a daisho.