We consider spontaneous generation of a high-order harmonic by an atomic be
am passing through a focus produced by two coherent pump waves propagating
at a small angle with respect to each other. It is demonstrated that, due t
o phase matching, the use of two coherent waves in a gas dispersive medium
allows the efficiency of high-order harmonic generation to be considerably
increased as compared with the case of a single wave with a doubled amplitu
de. Conditions permitting the optimization of harmonic generation are analy
zed.