While two earrings will conseculatively stack, two Atlas Armets won't. Same goes with Hero rings (although two Hero rings/Hero ring and Earring will have the same effec as two Earrings). Basically if your vigor is equal or above 128, then further Vigor will do nothing to your attack damage. There's a lot of stuff to consider, so I suggest you head over to GameFaq's FF6 section and look up the Algorithm's FAQ.