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The improvement of highly charged ion production with the use of two-frequency heating (TFH) in electron cyclotron resonance ion sources (ECRISs) was observed for the first time already in 1994, but the process has not been explained in details yet. The increase of the ionization rate because of the presence of two resonance surfaces instead of one is clear but not the relation between the two frequencies and the relative weight in terms of power. A series of numerical simulations about the heating process in ECR plasmas has been carried out at INFN-LNS, based on a single particle approach. They allow one to give a reasonable explanation of the TFH of the plasma electrons in terms of the microwave field distribution over the resonance surface, which changes with the excited mode, i.e., with the frequency. By analyzing the results, some considerations about the phase relationship between the electron's gyromotion and the electromagnetic wave can be done, taking into account the stochastic heating theory of ECR plasmas. The phase difference between the two waves in this case is shown to play a role. Finally, the analysis of the simulations' output will give the possibility to explain a great deal of experimental results concerning the multifrequency heating of plasmas in ECRIS.