In this paper, the aerodynamic performance of 2D tandem propeller-CoFlow Jet (CFJ) active flow control airfoils in cruise flight is numerically studied. The tandem propeller-CFJ airfoil system is an integrated system with upstream and downstream CFJ airfoils, each has a propeller mounted above the airfoil suction surface. The simulations employ 3D RANS solver with Spalart-Allmaras (S-A) turbulence model, 3rd order WENO scheme for the inviscid fluxes, and 2nd order central differencing for the viscous terms. The aerodynamic performance, energy expenditure, and flow field are compared between tandem wing systems with different downstream airfoil locations and angle of attack. The results show that the cruise efficiency is sensitive to the downstream airfoil angle of attack. Moreover, The wing-wing interaction effect on the aerodynamic performance is studied by comparing the optimum tandem propeller-CFJ airfoil system to the upstream airfoil and downstream airfoil only (without interaction) cases. It is observed that the wing-wing interaction increases the cruise efficiency significantly due to the energized flow of the propellers. The front propeller has a ”pushing” effect accelerating the flow to the downstream airfoil. The downstream propeller has a ”pulling” effect to accelerate the upstream flow. The downstream airfoil with higher angle of attack also induces an upwash and a higher circulation to the upstream airfoil. The optimal tandem airfoil interaction benefit observed in this paper appears to be attributed the proximity of the two airfoils, which are only one chord away horizontally with the rear airfoil 0.1 chord below the front one. This study may provide some guidance on the propeller interaction of a tandem wing. However, it does not include the important effect of the tip vortices and downwash of a 3D tandem wing system.