The electronic structures of two Mg- Si codoped configurations in wurtzite GaN were calculated by an ab initio "mixed-basis 1 norm conserving nonlocal pseudopotential" method. The results show that the charge densities redistribute and concentrate in the N-Mg bonds for the configuration with a local polarized field component that strengthens the polarizability. The tops of the valence bands are thus split widely and shifted up towards the conduction band. The N atoms bonding to the Mg atom are found to be as important as the Mg atom for the formation of the tops of the valence band. These electronic structure shifts can enhance the hole concentration about 103 times higher than that of Mg-doped GaN.