We investigated the influence of terbium and thulium trivalent rare-earth (RE) ions co-doping on the luminescent properties enhancement of LiMgPO (LMP) crystal host. The studied crystals were grown from the melt by micro-pulling-down (MPD) technique. Luminescent properties of the obtained crystals were investigated by thermoluminescence (TL) method. The most favorable properties and the highest luminescence enhancement were measured for Tb and Tm double doped crystals. A similar luminescence level can be also obtained for Tm, B co-doped samples. In this case, however, the low-temperature TL components have a significant contribution. The measured luminescent spectra showed a typical emission of Tb and Tm ions of an opposite trapping nature, namely the holes and electron-trapping sites, respectively. The most prominent transitions of D → F (550 nm for Tb) and D → F (450 nm for Tm) were observed. It was also found that Tb and Tm emissions show temperature dependence in the case of double doped LMP crystal sample, which was not visible in the case of the samples doped with a single RE dopant. At a low temperature range (up to around 290 °C) Tm emission was dominant. At higher temperatures, the electrons occupying Tm sites started to be released giving rise to emissions from Tb-related recombination centers, and emissions from Tm centers simultaneously decreased. At the highest temperatures, emission took place from Tb recombination centers, but only from deeper D level-related traps which had not been emptied at a lower temperature range.