We report on polymer liquid crystals with periodically oriented mesogenic side chains and demonstrate that the resulting two-dimensional polarization gratings multiplex-diffract the laser beam and convert the polarization state at the same time. Two-dimensional diffraction patterns with various kinds of polarization states can be successfully generated by designing a combination of one-dimensional polarization gratings. This study is a considerable advance towards the realization of highly functionalized passive optical devices that can control both the beam propagation direction and the polarization state.