Molecular dynamics (MD) simulations of molten LiCl/KCl and LiF/KF mixtures at 1030 K and molar fraction x = 0.5 and also of pure components LiCl, KCl, LiF, and KF have been performed. The aim of the MD simulations was to calculate the internal mobilities of the Li+ and the K+ cations in order to reproduce the Chemla effect, that is, the strong composition dependence of the cations' internal mobilities. In line with experimental findings, it has been found by MD simulations that the isotherms of the internal mobilities of chlorides and fluorides belong to different patterns. Whereas the more usual pattern of isotherms found in the chloride mixture can be explained by simple structural and dynamical properties depending most on first-neighbor anion-cation pairs, these do not explain why a distinct pattern of isotherms is found in the fluoride mixture. Structure and structural relaxation in the simulated systems are further discussed by calculating partial static structure factors, S-alphabeta(k), and intermediate scattering functions, F-alphabeta(k, t). Remarkable changes in the partial S-LiLi(k) are observed upon mixing; namely, an intermediate-range order develops in the mixtures as revealed by the presence of a pre-peak (or first sharp diffraction peak) at a wave vector k smaller than the main peak of S-LiLi(k). The intermediate-range order and an associated intermediate-range order dynamics are much more pronounced in LiF/KF than in LiCI/KCl, implying a strong coupling between the cations in LiF/KF. It is then proposed that structural and dynamical signatures for the different types of isotherms of internal mobilities in LiCl/KCl and LiF/KF are to be found in structural relaxation in a range beyond first-neighbor distances.