Two-Blade Fighting Style is one of four options for the Fighting Style ranger class feature. Rangers other than hunters or scouts who do not select the Beast Mastery class feature receive the Fighting Style class feature.
Rangers who select the Two-Blade Fighting Style option gain Toughness as a bonus feat. In addition, they can wield any one-handed weapon in their off hand as though it had the off-hand weapon property.[PH:104] Despite the name of the Two-Blade Fighting Style option, this benefit applies to all one-handed weapons, not just light and heavy blades.
Character optimization rates this as a good choice, due to the extra feat and allowing the dual-wield of one-handed weapons.[Charop Ranger]