The　CO2　reduction　reaction　(CRR),　which　is　key　to　addressing　global　environmental　and　energy　issues,　must　overcome　the　strong　C　=　O　bond　and　is　hindered　by　low　CRR　selectivity　and　activity.　Here,　we　unravel　the　ensemble　effect　of　catalytically　active　sites　for　promotion　of　CRR　while　suppressing　hydrogen　evolution　reactions　(HERs).　We　design　a　2D/2D　hybrid　consisting　of　an　ordered　assembly　of　nickel-rich　Ni-Mn　layered　double　hydroxide　(LDH)　nanosheets　(NSs)　and　oxygen　vacancies　(OVs)-rich　Mn3O4　(OVs-Mn3O4)　NSs.　On　its　own,　Ni-Mn　LDH　demonstrates　high　CRR　selectivity　but　mediocre　CRR　efficiency,　whereas　OVs-Mn3O4　gives　a　higher　efficiency　but　poorer　selectivity.　The　2D/2D　hybrid　inherits　advantages　from　both　components,　attaining　high　CRR　selectivity　and　markedly　higher　efficiency　than　Ni-MnLDH　and　OVs-Mn3O4.　Calculations　and　in　situ　Fourier　transform　infrared　spectroscopy　(FT-IR)　measurements　reveal　that　nickel　sites　in　the　2D/2D　hybrid　activate　CO2　to　generate　CO2　center　dot-　via　a　one-electron　injection,　whereas　OVs　in　OVs-Mn3O4　promote　light　harvesting　and　charge　transport　without　activating　unwanted　HERs.