Yang's claws don't really increase his attack power, and their effects don't stack with multiples of the same claw, so there isn't any reason to equip two of the same claw, really. The Cat Claw's claim to fame is it can put enemies to sleep and give a +5 Strength, which isn't really that much. Exploiting elemental weaknesses are a much more effective use of claws that the random chance to put an enemy to sleep and the +5 Strength. You also get the Fairy Claws in the Magnetic Cave, which are insanely useful against Ogres and Mad Ogres (even with Edge using them!) and also give +3 Strength. But really, two different elemental claws to hit as many weaknesses as possible for a given area are the way to go in general.