If you put a passive state ona skill, the state will become active when the actor knows the skill.
You can make a skill belonging to a skilltype that isn't usable by anyone, so it doesn't show up in menus, and have that skill have the dual wield passive state on it. I did something similar with passive skills in my own (permanently on hiatus) project, using the YEP skill learning system plugin, and a different skill equip system (which I think was made by moogleX?).
If you're not using that plugin, an ordinary skill learned on level up, but belonging to a skilltype the actor can't use, is still 'known', so should still confer the passive state, but the player will never see it.
~Ramza