While on the military base level where you have to rescue general Carington, there is a door on the left that leads to some kind of bedroom just before the room with the shaft leading to Carington's prison cell. Enter the room and take out the three SPADS soldiers in the room. At the opposite end of the room, to the right, is a locker standing by itself. Open it to get a 9 mm pistol, and the Dual Weapons skill will be enabled. From now on you will be able to use dual pistols and miniguns.