This paper describes a simulation approach for the modeling of tandem external gear pumps. A tandem gear pump is the combination of two pumps with a common drive shaft. The model presented in this work is applicable for both helical and spur gears. The simulation model is built on the HYGESim (HYdraulic GEars machines Simulator) previously developed by the authors for external spur gear units, and in this work it is properly extended to the capabilities of simulating helical gears. Starting directly from the CAD drawings of the unit, the fluid-dynamic model solves the internal instantaneous tooth space volume pressures and the internal flow following a lumped parameter approach. The simulation tool considers also the radial micro-motion of the gears, which influences the internal leakages and the features of the meshing process. The papers details how these aspects are modeled considering that both drive gears share a common shaft. The paper also presents the comparison of the simulation results with experimental data from tests performed on a physical tandem unit for automotive transmission systems, showing how the model is able to accurately predict the performance of both units.