In the design of a low bypass ratio fan stage, this paper links the through flow inverse design problems of the fan rotor, the core channel fan stator and the bypass channel fan stator together, as an organic whole, with the fan splitter aerodynamic design problem. It forms thus a unified through flow inverse problem for two-channel turbomachines. While an overall quasi-radial-equilibrium of the streamlines is iteratively achieved among five parts --- the fan rotor, the core channel fan stator, the bypass channel fan stator, the fan splitter and the position of the fan splitter, the blading processes for one fan blade, two fan vanes and one fan splitter ring are performed. This method designs in one pass a fan stage and a splitter with a precise bypass ratio. Its primary and key approach scheme is the introduction of a mass addition flow half way through the through flow streamline development. This scheme can serve as a new feature which can be inserted into those existing through flow programs. When subtracting a mass flow after adding a mass flow, alternatively, this scheme can also be used to treat the inverse design problem of a fan/compressor rotor blade with part-span shrouds.