The frequency-wave number (F-K) method and the spatial autocorrelation (SPAC) method can both be applied to estimate the subsurface structure by using array observations of long-period (1–10 sec) microtremors. Since these methods require more than four to seven sets of simultaneous recordings, the allocation of many resources, not only of instruments, but also of personnel, must be provided for the observations. To avoid this difficulty, we propose an alternative method based on the conventional SPAC method. It is called the two-site SPAC (2sSPAC) method and requires only two sets of seismographs under the assumption that the wave fields are spatially and temporally stationary. After formulating the analytical background of the 2sSPAC method, we use data from actual microtremors to discuss the limitations of the assumption and the applicability of this method through a comparison with conventional methods. This discussion leads to the conclusion that the 2sSPAC method provides reasonable values for phase velocities in the frequency range lower than about 1.0 Hz at the sites studied.