Understanding the structural and compositional sensitivities of the electrochemical CO2 reduction reaction (CO2RR) is fundamentally important for developing highly efficient and selective electrocatalysts. Here, we use Ag/Cu nanocrystals to uncover the key role played by the Ag/Cu interface in promoting CO2RR. Nanodimers including the two constituent metals as segregated domains sharing a tunable interface are obtained by developing a seeded growth synthesis, wherein preformed Ag nanoparticles are used as nucleation seeds for the Cu domain. We find that the type of metal precursor and the strength of the reducing agent play a key role in achieving the desired chemical and structural control. We show that tandem catalysis and electronic effects, both enabled by the addition of Ag to Cu in the form of segregated nanodomain within the same catalyst, synergistically account for an enhancement in the Faradaic efficiency for C2H4 by 3.4-fold and in the partial current density for CO2 reduction by 2-fold compared with the pure Cu counterpart. The insights gained from this work may be beneficial for designing efficient multicomponent catalysts for electrochemical CO2 reduction.