Direct electrochemical CO2 reduction reaction (CO2RR) is a process route to convert CO2 into valorized products via multiple electron-transfer reaction steps, with renewable electricity as the driving force. Catalyzing multi-electron reaction cascades at one and the same electrode, catalyst, or even molecular active site results in compromised, usually suboptimal overall reactivities. Owing to their design flexibility, however, tandem electrode and tandem cell designs raise overall efficiencies and product yields of multi-electron processes by enabling optimized operation points for individual process steps. To technologically implement a coupled tandem cell design for the CO2RR, CO-rich streams supplied by a "CO2-to-CO" electrolyzer (Cell 1 in Figure ) will be fed into the "CO-to-X" electrolyzer (Cell 2 in Figure ) for downstream production of C2+ chemicals and fuels (Figure ). To make this tandem cell scheme viable, a faradaic 100% efficient CO2-to-CO electrolysis cell 1 operating at industrially relevant current densities, at high CO space time yield, Y SP CO, at high catalytic CO2 utilization,  cat CO 2 , at high total energy efficiencies, EE, and high pass conversion, SP, must be designed at very low, near unity stoichiometric CO2 excess, . While catalyst screenings and idealized cell conditions can optimize each of these perfomrnace parameters (Supplementary Table ) individually, the simultaenous optimization and analysis of all this performance parameters in operating CO2-to-CO electrolyzers has remained elusive. Despite near 100% faradaic efficiency for CO, the exit flow compositions of previously reported CO2-to-CO electrolyzers have typically remained very CO2 rich, as the cells were not operated at low enough (<2) stoichiometric ratios, , nor high enough single pass conversions, SP, nor high enough catalytic CO2 utilizations,  cat CO 2 . As a result, previously published CO2-to-CO electrolyzer cell designs failed to offer tandem design advantages over pure CO2 feeds to Cell 2. This is why new cathode catalysts and cathode layer designs, as well as a cell-level diagnostic analysis and control of cell parameters such as the stoichiometric ratio, , single pass conversion, SP, or the CO2 utilization,  cat CO 2 , of cell 1 are required to fully unfold kinetic benefits of the combined coupled cascade. While much past work was dedicated to understand, and design the atomic-scale catalytic active sites of individual CO2RR catalyst materials, future work need to place equal focus on a thorough diagnosis and design of the dynamic operation points of the triple phase cathode layers of CO2-to-CO electrolyzers. This work contributes to that.