Two nearly noninteracting π and σ bands of different dimensionality are key ingredients in MgB₂. Critical temperature and other superconducting properties of two-band superconductor depend on the doping level and on the interband and intraband scattering and can be modified by chemical substitutions. Due to strongly anisotropic character of MgB₂, single crystal studies of its anisotropic properties are very important. In the crystals codoped simultaneously with Li-C and Li-Al one can expect compensation of electron doping with C or Al and increase of T_c. Following this strategy, we synthesized a series of Mg_1-xLi_x(B_1-yC_y)₂ and Mg_1-x(Al,Li)_xB₂ single crystals under high-pressure/high-temperature conditions. Our experimental results show that holes introduced with Li in Li-C codoped crystals cannot counterbalance electrons coming from C. The T_c reduction in Li-C codoped crystals is a sum of T_c reduction of separate Li and C doping. The possible reason of it can be that holes added by Li occupy π band and do not compensate the electrons from C, which fill the σ band. The comparison of the results on Mg_1-x(Al,Li)_xB₂ with previous experiments on Mg_1-xAl_xB₂ crystals shows that T_c reduction is a function of Al concentration alone. This finding suggests that effect of Li in MgB₂ is far from trivial and not yet understood. The total influence of codoping on T_c of MgB₂ should be understood as a result of the competition of two effects: the first one is a codoping effect related to the changes of the carrier concentration and the second one is related to the introduction of new scattering centers, leading to the modification of the interband and/or intraband scattering.

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