With two pieces of phase gratings stored in one point of photorefractive materials, the analytic expressions for the first two higher-order harmonics of the stationary space-charge field are presented by using “the hopping model” of Feinberg. The results indicate that the fundamental and second harmonic of the space-charge field of one of grating are coupled with the second and fundamental harmonic of the other grating, respectively. The positions of the maximum values of the normalized fundamental and second harmonic of the space-charge field versus the normalized grating vector (k/k0) in two pieces of grating are deviated from that in single piece of grating. At the same time, it is found that the degree of deviation of the spatial second harmonic of the space-charge field more strongly depends on the difference of normalized grating vector (k/k0) of the two pieces of gratings than that of fundamental harmonic. Besides, the degrees of deviation for both harmonics do not depend on the modulation depths.