Tandem catalysis, capable of decoupling individual steps, provides a feasible way to build a high-efficiency CO₂ electro-conversion system for multicarbons (C₂₊). The construction of electrocatalytic materials is one of focusing issues. Herein, we fabricated a single atom involved multivalent oxide-derived Cu composite material and found it inclined to reconstruct into oxygen-deficient multiphase Cu based species hybridized with monatomic Ni on N doped C matrix. In this prototype, rapid CO generation and C−C coupling are successively achieved on NiN₄ sites and surface amorphized Cu species with defects, resembling a micro-production line. In this way, the in situ formed tandem catalyst exhibited a high Faradaic efficiency (FE) of ~ 78% for C₂₊ products along with satisfactory durability over 50 h. Particularly, the reconstruction-induced amorphous layer with abundant asymmetric sites should be favorable to improve the ethanol selectivity (FE: 63%), which is about 10 times higher than that of the non-tandem Cu-based contrast material. This work offers a new approach for manipulating tandem catalyst systems towards enhancing C₂₊ products.