Simple: Allow gear that enhances Dual Wield to grant the trait.
As far as I know, Dual Wield is the only job trait that is not automatically granted from equipment with "Enhances 'job trait'" in its description. (Keep in mind I'm talking about job traits here, not job abilities. I know that equipping an item that enhances a job ability does not grant access to that ability, nor do I think it should)
This would give Dual Wield to anyone who wants it (since there is equipment that enhances Dual Wield that is equipable by all jobs) without making any changes to any jobs (which could potentially alienate some jobs if they were still left out of the change).
I think this would be fair and not terribly game breaking since you would still have to make an accommodation to get the trait, that is; you would have to equip an item that may not ordinarily have the best stats for that slot.