I was actually a bit surprised coming into this game to find out that NINs didn't equip anything in their offhand slot. An issue I already see is for those who don't mind dual wielding twin blades would be screwed for anything that is unique. Think of the two ring slots when equipping blues or certain greens. Otherwise, it could easily be handled in the same manner that PLDs are with their swords and shields.

Not likely to happen though. The closest would probably be blades that are intentionally different by design. However, you wouldn't be able to choose the second.