Does the two-handed ability increase all damage done by the character as long as a staff is equipped on him, or only damage done by the default staff skill, not actual spells?
A quick check confirms that there's no difference for me with any spell cast with points in two-handed. It *will* improve the damage of your default staff skill, but I really don't think you should be using that very much after the first hour or two of the game.
That skill is really for the direct damage-dealers - the ones who hit with weapons. You won't need it for any character that relies primarily on spells.
