I think Firion would be better off equipping a second weapon type instead of two fists, wouldn't he? I don't think he gets the bonus from his passive twice from two of the same weapon.
Champion's Belt
Stats: DEF+30, ATK+30
Champion's Belt (Ability)
Effect: Increase ATK/DEF (20%) when equipped with a fist weapon
Depends how the Champion's Belt's work. I mean if its
2x 20% ATK to Knuckle a
2x 20% ATK to Knuckle b
Then equipping 2 Knuckles would be better than slapping some random sword which gives 20% or 30% ATK.
If you get the bonus ATK & DEF only from one CB then sword could be better.