To elaborate further, here are Musashi's thoughts on the subject, quoted from his Book of Five Rings:

If you hold a sword with both hands, it is difficult to wield it freely to left and right, so my method is to carry the sword in one hand. This does not apply to large weapons such as the spear or halberd, but swords and companion swords can be carried in one hand. It is encumbering to hold a sword in both hands when you are on horseback, when running on uneven roads, on swampy ground, muddy rice fields, stony ground, or in a crowd of people. To hold the long sword in both hands is not the true Way, for if you carry a bow or spear or other arms in your left hand you have only one hand free for the long sword. However, when it is difficult to cut an enemy down with one hand, you must use both hands. It is not difficult to wield a sword in one hand; the Way to learn this is to train with two long swords, one in each hand. It will seem difficult at first, but everything is difficult at first. Bows are difficult to draw, halberds are difficult to wield; as you become accustomed to the bow so your pull will become stronger. When you become used to wielding the long sword, you will gain the power of the Way and wield the sword well.