The excited electronic states of nearby molecules are in general coupled via Coulomb interaction even in the absence of wavefunction overlap. When two different organic molecules having a nearly resonant excited state are close enough, the dipole-dipole interaction can significantly affect their optical response. Even though they do not chemically interact, concerning their coupling to light, such molecules do not act independently, but rather as a 'virtual heterodimer' the response of which stems from, but is different from that of each molecule alone. We discuss here a simple and general model to estimate their resonant nonlinear susceptibilities.