Watch Darker than Black 2 : Gemini of the Meteor OVA full episodes English dub.
Other title: Darker than Black: Kuro no Keiyakusha Gaiden, Darker than Black: Ryuusei no Gemini Specials, Darker than BLACK 2 OVA, DTB
Synopsis: Tells the story of what happened to Yin and Hei between the first and second series.
More infor: Myanimelist, Wikipedia