Example: Joyeuse will never attack more than 2 times, and the 2nd attack will never be subject to Double Attack. Furthermore, Kraken Club is never affected by Double Attack if its "Attacks 2 to 8 Times" effect proc's, though it will increase the amount of hits per round.
Double Attack can trigger on both weapons when Dual Wielding, both hands when using H2H weapons and both attacks on virtue stone weapons.
Example: Faith Baghnakhs can produce an additional attack on both hands (consuming 2 virtue stones), and each of these 4 attacks are subject to Double Attack.
When using a normal H2H weapon, the double attack will only appear on the screen on the left hand. When both fists trigger a Double Attack, both are done with the left hand on the screen. When using Faith Baghnakhs, double attack from extra swings can appear on left and right hands.