Dual wield when?

[Niwashi]

ninjas don't count as they use daggers.
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I would love it if samurai dual wielded katanas but im pretty sure if it comes, they only use 2h katanas.

Ninja sometimes use daggers (and in the lower levels, as rogue, the class quests discuss their weapons as being daggers), but a lot of their later weapons are swords. And samurai would probably use 2H katanas (or ōdachi) to distinguish them from Ninja who can already dual wield 2 katanas.

DO you really want to hear the cries and complaints from people who have to farm 2 weapons just because they dual wield?

Would it be that different from paladins, who already have an extra slot to fill? Sure, theirs is a shield rather than a second weapon, but it's pretty important for them. If there were a dual wielding class that used separate main and off hand weapons (rather than the two in one pattern that Ninja and Monk do) they'd simply be half the cost or twice as easy to get to counter the fact that they'd need two of them, much the same as a paladin's sword and shield together cost the same as a weapon for any other job (at least in places where SE can directly control the cost, like tome weapons).

Never. There is no such thing as "proper" dual wielding. If you look historically - back when they actually used swords, axes, etc. to fight - nobody ever[*] used two weapons at once. When possible weapon+shield or (rarely) a 2h-weapon was much preferred.
[*] There is actually one well-known exception. The use of rapier+dagger was not uncommon during the renaissance, but that is just about the only historically documented occurence of using two weapons at once.

Well, that's not quite the only time a dagger was paired with another weapon, but was probably its most widespread use, and fits a broader real-world pattern. Pretty much any time two weapons were wielded at once, the off-hand weapon would be something small. Wielding a full-sized weapon in each hand seems to be a purely fantasy creation (though a quite popular one, since it looks badass, and in fantasy that counts for more than the practical considerations of how cumbersome it would be).

I think it would be interesting if the game had a dual wielding class where the main hand weapon and off hand weapon were two different classes of items, like main hand sword and off hand dagger. I don't think it's likely, though, since it would double the number of weapons SE would have to design for the job. More likely would be other jobs that dual wield the way monks and ninja do, with a pair of matching weapons as a single item (dual axes, maybe).