Map the Fireball spell to the quick menu's X button. With the bow in-hand, hold the left trigger to target an enemy, then hold the right trigger to access your quick menu, then hold the X button. The Fireball spell should now start to charge in your hand. While still holding left trigger and X, release the right trigger. Instead of throwing the fireball or cancelling the spell, your character will begin to draw an arrow. When you've pulled back all the way, release the X button and you'll shoot both the arrow and the fireball at roughly the same time. This glitch may work with other spells, preferably anything with a charge/release effect such as Fireball or a repeat-while-held effect such as Lightning.