I've read mixed things about this so I'd like to clear them up. People ingame refuse to answer this question for some reason so I decided to try here. How the hell do you dual-weild daggers?
I've read that it "may" be only certain daggers that are dual-wieldable, or that you have to dual-class Warden. Which is it? or is it neither?
As others have said, Off-hand Training is a Rogue general skill. Depending on the other class(es) you pair with Rogue (R/x or x/R), you can dual-wield daggers, 1-H swords, or 1-H axes, based on the second class you choose. (Technically, you could dual-wield a Wand and Talisman or a pair of Wands, as a spell-caster/Rogue, but...don't get into melee range!)
As KatalanOrk noted, there are few offhand-specific weapons. All daggers and most 1-H swords and axes can be used offhand.
Off-hand Training becomes available at Rogue level 16.