An analysis of a multiplex computer-generated hologram (Twin-CGH) is presented, with the extended absolute measurement of spherical surfaces. By means of the five-position test, the pattern error and figure error of Twin-CGH operated in spherical modes with two different optical power are separated respectively, and internal relations between two different holograms are proved. This relations is based on the relations of the spatial frequencies corresponding to two different phase functions. In this article, two kinds of errors in different modes are derived from each other. The derivative results are compared with experimental ones, and the residual error are less than λ/40 (PV, λ=632.8 nm). The results demonstrate the feasibility of error transfer in Twin-CGH. It shows that the aspherical reflective surface with known errors can be obtained using a Twin-CGH which reconstructs a spherical wavefront as well as an aspherical wavefront. The aspherical reflective surface can calibrate null grating.