Diffusion-doping is an effective, practical method to improve material properties and widen material application. Here, we demonstrate a new physical phenomenon: diffusion control of an ion by another in LiNbO3 and LiTaO3 crystals. We exemplify Ti(4+)/X(n+) (X(n+) = Sc(3+), Zr(4+), Er(3+)) co-diffusion in the widely studied LiNbO3 and LiTaO3 crystals. Some Ti(4+)/X(n+)-co-doped LiNbO3 and LiTaO3 plates were prepared by co-diffusion of stacked Ti-metal and Er-metal (Sc2O3 or ZrO2) films coated onto LiNbO3 or LiTaO3 substrates. The Ti(4+)/X(n+)-co-diffusion characteristics were studied by secondary ion mass spectrometry. In the X(n+)-only diffusion case, the X(n+) diffuses considerably slower than the Ti(4+). In the Ti(4+)/X(n+) co-diffusion case, the faster Ti(4+) controls the diffusion of the slower X(n+). The X(n+) diffusivity increases linearly with the initial Ti-metal thickness and the increase depends on the X(n+) species. The phenomenon is ascribed to the generation of additional defects induced by the diffusion of faster Ti(4+) ions, which favors and assists the subsequent diffusion of slower X(n+) ion. For the diffusion system studied here, it can be utilized to substantially shorten device fabrication period, improve device performance and produce new materials.