I may not know much from a programming stand point, but I would imagine building from what they have about shield, would probably be easiest to make a sub set of dual weapons that goes into the shield category like how swords, guns, and bombs go into the weapon category but sometimes easy isn't best.

---key points---

-when using a weapon as a shield the shied bar is 80% smaller than normal and takes longer to recharge, especially when broken.

-using weapons as a shield results in lackluster defense, just a little bit of normal resistance.(some weapons have better resistance than others, namely big blades like troika/caliber lines, antigua's or the big angry bomb)

-The shield bash does the weapons charge move immediately but fully depletes the shield bar, breaking it.

-instead of the shield bump that the regular shields have, they have a parry of sorts that deals about half the regular damage of the weapon.(swords swing, guns shoot with extra damage, and bombs explode with a greatly reduced range)

--Unique Variants--

-attack speed makes you do the "parry" move faster.

-charge time reduction makes the shield bar recharge faster.