Two femtosecond four-wave-mixing techniques carried out using the photon echo pulse sequence with positive or negative time delays are shown to provide a direct probe for two-exciton dynamics in light-harvesting antennae of photosynthetic bacteria and green plants. The spectroscopic signatures of intramolecular and interaction-induced (intermolecular) two-exciton states are analyzed using a collective excitonic oscillator representation of the nonlinear optical response.