We have theoretically shown that efficient generation of multi-electron-hole pairs by a single photon observed recently in semiconductor nanocrystals1-4 is caused by breaking the single electron approximation for carriers with kinetic energy above the effective energy gap. Due to strong Coulomb interaction, these states form a coherent superposition with charged excitons of the same energy. This concept allows us to define the conditions for dominant two-exciton generations by a single photon: the thermalization rate of a single exciton, initiated by light, should be lower than both the two-exciton state thermalization rate and the rate of Coulomb coupling between single and two exciton states. Possible experimental manifestations of our model are discussed.