A dual-frequency gyrotron has been developed within the context of the recent Tokamak à Configuration Variable (TCV) upgrade. The gyrotron is designed to generate a 1 MW, 2 seconds RF wave at 84 or 126 GHz. Before integrating the gyrotrons in the TCV tokamak ECRH system, an extensive characterization of their behaviour has been performed. This paper focuses on presenting the results of these experiments at the two operating frequencies. The power measurements are systematically compared with numerical simulations. This comparison highlights the validation of numerical codes and the effect of After Cavity Interaction (ACI), a crucial factor that must be considered for achieving a good agreement between theoretical predictions and experimental results.