The two-piece or two plait braid is known and loved as the fishtail braid. Blake Lively is wearing a ponytail wrap around hair extension that is braided into her own hair thus creating a thick, disheveled fishtail braid.
To create this look, a clip in ponytail was attached to Blake Lively's natural ponytail and then married/braided together. The disheveled look was created by backcombing the plaits slightly before braiding. I would also "guess" that there is either a sea-salt spray or some type of texture product to hold the braid in place.