they are like a twin set of lightning claws. they give you +1 att. for the double weapon, but you attack with just one of each each round of melee. it would be the same being equipped with a powermaul and a powersowrd. you can't have both bonuses (+2 str and AP3), you just use one weapon per subphase, with +1 attack. they have the same stats (+1 str ap 3 etc.) and thus it doesnt matter wich one you use. you get the same once, with +1 att.