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 (follows a Lead Attack).
Successful Dual Attacks always result in a double strike, doubling the damage done.
For example, Twisting Fangs, at 16 Dagger Mastery, will show up as +19 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 +38 damage.
It is important to note that dual attacks do not necessarily always hit twice. There is about a 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, your Twisting Fangs may only do just +19 damage because you were interrupted before you could get the other +19 off.
Dual attack effects (such as conditions) go off ONLY on the first hit. So if you were interrupted half way through Twisting Fangs, you'd lose that extra +19 damage attack, but the target would still be afflicted with Bleeding and Deep Wound because of the first hit.
For a list of all Dual Attack skills see: Dual Attacks