A novel CO2-stable and reduction-tolerant Ce0.8Sm0.2O2−δ–La0.9Sr0.1FeO3−δ (SDC–LSF) dense dual-phase oxygen-permeable membrane was designed and evaluated in this work. Homogeneous SDC–LSF composite powders for membrane fabrication were synthesized via a one-pot combustion method. The chemical compatibility and ion interdiffusion behavior between the fluorite phase SDC and perovskite phase LSF during the synthesis process was studied. The oxygen permeation flux through the dense dual-phase composite membranes was evaluated and found to be highly dependent on the volume ratio of SDC and LSF. The SDC–LSF membrane with a volume ratio of 7:3 (SDC70–LSF30) possessed the highest permeation flux, achieving 6.42 × 10-7 mol·cm–2·s–1 under an air/CO gradient at 900 °C for a 1.1-mm-thick membrane. Especially, the membrane performance showed excellent durability and operated stably without any degradation at 900 °C for 450 h with helium, CO2, or CO as the sweep gas. The present results demonstrate that a SDC70–LSF30 dual-phase membrane is a promising chemically stable device for oxygen production and CO2 capture with sufficiently high oxygen permeation flux.