In this study, biochars obtained by either direct carbonization or hydrothermal treatment of corn stems were used as precursors of two series of activated carbons (ACs). The pore size distributions (PSDs) of biochars and ACs were evaluated from N2 and CO2 adsorption isotherms using models based on the two-dimensional version of the non-local density functional theory (2D-NLDFT). We showed that more detailed carbon PSDs might be obtained from simultaneous (dual) gas analysis of both N2 and CO2 isotherms than from single isotherms. The dual gas method showed a peak at about 0.4 nm which was not detected by the single N2 analysis. This fact is related to the restricted diffusion of N2 into very narrow micropores at low temperatures and pressures. By modifying the lower pore width limit wmin in the nitrogen model, we obtained an excellent fit of the dual model to both isotherms for all studied samples, which demonstrates the reliability and robustness of this refined method. Finally, we demonstrated that this approach would allow measuring N2 isotherms at relative pressures starting at about 0.001 rather than at 10−6, which would save time without losing resolution of the calculated PSD.