The only job that should get true 2H Weapon dual wield is Warrior, and the ability for it in the FF franchise is called "Monkey Grip."
Meanwhile, samurai should get an ability that allows them to -equip- a 1handed weapon into their offhand slot and have it appear on them as they fight with their Great Katana. They can't use it, but then you can an additional ability that allows them to access weaponskills from that weapon and use them with Great Sword.
Its the best of both worlds. You keep Samurai from looking like complete idiots dual wielding Great katana (which is a trashy, "Omg, so cool!" movie gimmick at best), but you allow them access to additional weapon skills by embodying the classic image of a Samurai with a Katana and a Wakizashi.