There's a very stupid bug/glitch that I've found, where if you assign a spell to your first hand there's no way to unequip it or equip another spell there. If you try to select/unequip the same spell, it just toggles between one-handed and two-handed, and if you select another spell it always puts it in the left hand. In order to change your right-hand spell you need to go to items, equip and unequip a weapon to leave your right empty, then pick the spell you want in your right hand.
Pain in the ass for a mage character.