An electronic system externally distorted by photons or particle impact can integrally respond by the simultaneous emission of two electrons into the vacuum. The spectra of these correlated electron pairs carry various information depending on the scattering geometry and the initial conditions from which the pairs originated. In this work, the dominant ejection process of the electron pair in the back-reflection mode are investigated. Diffraction conditions, equivalent to the von-Laue conditions for the Bragg scattering, are derived. In addition, the simultaneous emission of an electron pair upon single photon absorption is discussed.