Do the stat bonuses and passive special attack chances from weapon skills double when dual-wielding two weapons of the same type? Do they stack when dual-wielding with two different weapon types (i.e. a mace and a staff)? What about when using a weapon skills active attacks?
If you are dual wielding you get all of the applicable weapon skill bonuses for each weapon. Therefore if you wield two weapons of the same type, you get twice the bonuses from your weapon skill tree.
I don't yet know about special attack chance, but I'll keep an eye out and update this answer if I find out.