You can use a double weapon to fight as if fighting with two weapons, but if you do, you incur all the normal attack penalties associated with fighting with two weapons, just as if you were using a one-handed weapon and a light weapon. The light weapon end is thus usable with 'Weapon Finesse' and similar feats. You can choose to wield one end of a double weapon two-handed, but it cannot be used as a double weapon when wielded in this way-only one end of the weapon can be used in any given round. If a double weapon is sundered, the creature inflicting the sunder chooses the end that they are sundering, but only one end of the weapon actually suffers the durability damage of the maneuver. To completely break a double weapon, both ends must be Sundered until the weapon's durability is exceeded, effectively doubling the durability of the weapon.