To get the Twinned Armor set, you'll need to either kill D when you first meet him in Limgrave, or simply progress through Fia's questline until you receive her dagger. Hand the dagger to D, then reload the area and head past Hewg to find a newly opened room where D lies dead on the floor. You can then pick up D's Bell Bearing, along with the entire Twinned Armor set.