Thermoelectric materials enable the direct inter-conversion between electrical energy and thermal energy. However, the conversion efficiency is limited by complex interdependent thermoelectric parameters, while the high performance thermoelectrics should simultaneously possess excellent electrical transport properties and poor thermal conductivities. The diamond-like compound Cu₂SnSe₃ is a promising middle-temperature thermoelectric material. In this work, the phase (Cu₂Sn_0.93Ag_0.07Se₃) with excellent electrical transport properties and the phase (Cu_1.91Ag_0.09SnSe₃) with poor thermal conductivities were obtained just through Ag doping on the Sn and Cu sites, respectively. Meanwhile, their Seebeck coefficients were also quite different. To combine their advantages, the composites of Cu₂Sn_0.93Ag_0.07Se₃ and Cu_1.91Ag_0.09SnSe₃ were fabricated through mechanical mixing and sintering. Benefited from the same crystal structure and the similar lattice parameters for these two phases, the small-mismatch phase interface is supposed to scatter phonons with little influence to the electrons, especially at high temperature. Therefore, the thermoelectric performance is improved due to the synergistically optimized electrical and thermal transport properties, which are well supported by the effective media theory for the composite.