They could do it easily, by creating a swordshield item that is consumed like a coffer, for every dropable sword and shield in the game, but this would create a lot of redundant items. I'm personally of the opinion that they should just combine swords and shields into a single swordshield item, glamours be damned; or combine them into a single item and then implement a new offhand glamour system that lets PLD MNK NIN RDM DNC glamour their offhand model using a second weapon as glamour.