The Ritchey-Common test is a widely used method for testing flat mirrors with a larger reference spherical mirror. However, with the increase in the size of the flat mirror, the fabrication of the spherical mirror becomes more time consuming and expensive. In this study, we developed a novel technique to test a large flat mirror with a smaller reference sphere using a dual subaperture stitching (DSS) method. One part of the DSS technique is a modified Ritchey-Common test, which uses a reference sphere smaller than the flat mirror. The other part involves scanning along the centerline of the flat mirror. The former can be used to determine the surface form error (SFE), except for rotationally symmetric components, such as power and spherical aberrations, which can be measured by the latter. To perform the stitching process using a smaller reference sphere, the flat mirror was repeatedly rotated by a fixed angular step. One of the advantages of the rotation of the flat mirror is that it can be used to identify the errors resulting from the reference sphere, which do not vary during the rotation of the flat mirror. We verified the DSS method using a 152 mm diameter optical flat mirror and determined the root-mean-square (rms) measurement error to be only 0.2 nm, which was comparable to the error of the full-aperture interferometric measurement. In addition, we tested a 1.2 m diameter flat mirror with a reference sphere with an aperture of 0.8 m and measured its SFE to be 11.9±0.5nm rms.