The two-photon absorption in a two-atom cavity quantum electrodynamics (QED) system is studied in detail. In free space, two atoms of different frequencies cannot be excited at the same time because of the quantum interference effect. In the strongly coupled cavity QED system, however, the coupling between the atom and the cavity field leads to new transition channels, which makes diatomic excitation possible. The photon excitation spectrum of a two-atom cavity QED system is elaborately studied by the master equation of numerical simulation and is compared with the two-photon excitation spectrum, which proves the possibility of two-photon excitation. Further analysis of the second-order correlation function and the diatomic excitation probability of the photon reveals the statistical properties of the photon in the cavity and the physical mechanism of diatomic excitation.