You can wield two weapons or a double weapon without penalty.
Effect: When you attack with two weapons, or with both ends of a double weapon as part of a Full Attack action, you take no penalty on your attack rolls. You only gain this benefit if you are wielding a weapon with which you are proficient.
Normal: If you use a Full Attack action to make more than one attack on your turn, you take a -10 penalty on all attack rolls for the round.
Additional Dual Weapon Mastery III Effects Edit
Reference Book: Star Wars Saga Edition Starships of the Galaxy
Vehicle Weapons cannot be used in one hand, so this Feat has no effect on them.