Greetings.
I have uploaded a new version of the dual wield plugin which fixed a bug, and also added the two features we talked about here.
<Forbidden wtypes: x, y, z> can now be placed on shields, allowing you to specify weapons that cannot be used together with specific shields.
I also added a plugin parameter to show forbidden weapons in the mainhand slot, if they're being forbidden by the item in the offhand. Equipping a forbidden weapon into the main hand when the offhand forbids it will unequip the offhand item, which is functionally the same thing it does when you equip a two-handed weapon in the offhand.