Using a beam-gas technique, double electron transfer cross sections of non-dissociative C₂^- and C₂H^- ion formations with a Cs target were evaluated to be 7.03 × 10^-18 and 1.87 × 10^-18 cm² at laboratory energy of 3.0 keV, respectively. The cross sections of C₂^- and C₂H^- ion formation decreased with the decrease of the laboratory energy, becoming 2.15×10^-18 and 3.41 × 10^-19 cm² at 0.7 keV, respectively. The cross sections of double electron transfer with K or Na targets were estimated to be less than 2 × 10^-20 cm² at collision energy between 0.7 keV and 3.0 keV. The energy dependence and the magnitude of the cross sections were compared with an adiabatic criterion and Landau-Zener approximation. The Landau-Zener approximation gave the cross sections which were one order of magnitude larger than the experimental value. This difference between the calculated value and the experimental value would be explained from the influence of intermediate pseudopotential crossings between an ion pair state and excited ion molecule states.