See also: Feats
You are adept at fighting with two weapons and double weapons.
Effect: When you attack with two weapons, or with both ends of a double weapon as part of a Full Attack Action, you take a -5 penalty (Instead of a -10 penalty) on all attack rolls until the start of your next turn. You only gain this reduced penalty 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 I 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.