It was said an interview that they cannot make Ninja's daggers to have different appearance on each hand, because they don't make a main and an off-hand dagger, but a single model that is copy-paste into another hand. Other items like Red Mage's focus, Samurai's sheath... also share the same fate.



To cater your request, this will require SE to make several sets of off-handed items so that they can be glammed separately like a shield, thus inflating the number of items which SE has been already trying to cutting down.