We provide a Landau theory of the coupled charge density wave and spin density wave order parameters to investigate the possibility of coexistence or competition between the two ordering processes occurring in the two-dimensional doped antiferromagnet. We find that the two ordering processes can coexist only in a certain region of coupling parameters, and the one with lower transition temperature can only coexist together with the other one with higher transition temperature and cannot be stabilized separately. Phase diagrams in different situations are given.