Many investigations have defined Smith-type diagrams to guide the preliminary designs of conventional axial compressor stages on the choice of loading, flow coefficient, and degree of reaction. However, the recent development of unconventional axial compressor stages with tandem vanes has not been accompanied by similar studies aimed at tailoring existing correlations to the new type of vanes. While it is clear that axial compressor stages with tandem vanes operate in higher working ranges than conventional stages, it is less clear how the choice of reaction affects the aerodynamic behavior of such setups. For this purpose, this paper numerically investigates a low-speed axial compressor stage with different degrees of reaction for increasing loading levels. The metal angles of the unshrouded rotor and the shrouded stator are modified to ensure that the other design parameters of the stage, namely the work and flow coefficients, are kept constant, and that the influence of the degree of reaction is isolated. The investigation begins with Q2D simulations of the reference midspan aerofoils. It then extends to a 3D configuration, while maintaining the radial distribution of the aerofoil parameters from the reference 3D blades. New correlations are presented, aiming to show how the performance of the stage in terms of efficiency, total pressure losses, and loading coefficients of the vanes are influenced by the different degrees of reaction investigated. This paper, therefore, provides insight into the preliminary choices of parameters for the design of axial compressor stages with tandem vanes.