A double khopesh is a double weapon. You can fight with it 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. A creature wielding a double khopesh in one hand can't use it as a double weapon-only one end of the weapon can be used in any given round. A double khopesh can be used to make trip attacks like a normal khopesh.