We present a theoretical investigation of the nonlinear phenomenon of spatial frequency mixing in photorefractive materials. In particular, we study the kinetics of the second harmonics and the sum and difference (combinational) gratings when two photorefractive gratings are recorded in the material. The physical origin of the new gratings is extensively discussed. The formalism is applied to investigate multiple recording in LiNbO3 as a material relevant for applications. The influence of the multiple-recording method (either sequential or simultaneous) on the generation of second-order gratings is analyzed. We found remarkable differences in the kinetics of these gratings depending on the multiplexing procedure. Our theoretical predictions are in good agreement with a number of previously reported experimental results.