Strong-field double ionization of atoms in a non-sequential regime produces longitudinal ion momentum distributions with a characteristic double-peak structure. At 800 nm laser wavelength in Ne²⁺ the structure is very pronounced with a well-resolved dip at zero momentum, while for Ar²⁺ the dip is very shallow, possibly indicating different mechanisms in the two atoms. We investigated the source of this difference by measuring longitudinal momentum distributions of Ne²⁺ and Ar²⁺ ions at different laser wavelengths (485, 800, 1313 and 2000 nm) and intensities. The shapes of the experimental momentum distributions for the two atoms exhibit strong dependence on laser wavelength: for both the dip becomes more pronounced at longer wavelengths. At 1300 nm the longitudinal momentum spectrum for Ar²⁺ is similar to that of Ne²⁺ at 800 nm. On the other hand, the Ne²⁺ spectrum measured at 485 nm has the same shape as that of Ar²⁺ at 800 nm. This observation indicates that the difference between Ne and Ar observed at 800 nm should not be attributed solely to differences in relative electron impact ionization and excitation cross-sections of the two atoms. It is, to a larger extent, due to the interplay between the ponderomotive energy of electron and the ionization potentials of the target atom.