Dual Attacks are the final and most powerful attacks in the dagger attack sequence of the Assassin.
To activate a Dual Attack, the Assassin must first land a successful Off-Hand Attack (which often follows a Lead Attack).
Successful Dual Attacks always result in a double strike, doubling the damage done.
For example, Twisting Fangs, at 16 Critical Strikes, will show up as +21 damage if you hover your mouse over the skill in-game. But since you'll be hitting twice, you'll actually do a total of +42 damage.
It is important to note that dual attacks do not necessarily always hit twice. They have independent miss chances, are blocked and evaded independently, and there is about 1/8th of a second time period between the first and second attack, which means that it can actually be half way interrupted. For example, if you used Twisting Fangs against someone protected by Shield of Judgment, it would only do +21 damage because you would be knocked down before you could get the other +21 off.
Dual attack effects (such as conditions) go off on both the first hit and the second hit, but don't stack. So if you were interrupted half way through Twisting Fangs, you'd lose that extra +21 damage attack, but the target would still be afflicted with Bleeding and Deep Wound because of the first hit. Or, in cases where the enemy had Riposte up, you'd miss the first hit, but the second would still put Bleeding and Deep Wound on them.
For a list of all Dual Attack skills see Dual attack skills quick reference or Category:Dual Attacks.