A simple way is the phoenix's ability that fires fireballs, but it's not really an attack.
A more complicated way, you can use a dummy (with locust ability) - an invisible, invincible, unselectable unit, to do the second attack.
Track the main unit's position and targets with a trigger, move the dummy to the main unit's position, and order it to attack the same unit (or not). You would also need to check if the main unit is moving or stunned and pause the dummy so it doesn't attack, and check for any attack-affecting buffs on the main unit and apply them to the dummy. Dummies can't be targeted but you can give them item abilities for +/- attack or attack speed.
