Extending the scheme developed for a single mode of the electromagnetic field
in the preceding paper ``Structure of multiphoton quantum optics. I. Canonical
formalism and homodyne squeezed states'', we introduce two-mode nonlinear
canonical transformations depending on two heterodyne mixing angles. They are
defined in terms of hermitian nonlinear functions that realize heterodyne
superpositions of conjugate quadratures of bipartite systems. The canonical
transformations diagonalize a class of Hamiltonians describing non degenerate
and degenerate multiphoton processes. We determine the coherent states
associated to the canonical transformations, which generalize the non
degenerate two--photon squeezed states. Such heterodyne multiphoton squeezed
are defined as the simultaneous eigenstates of the transformed, coupled
annihilation operators. They are generated by nonlinear unitary evolutions
acting on two-mode squeezed states. They are non Gaussian, highly non
classical, entangled states. For a quadratic nonlinearity the heterodyne
multiphoton squeezed states define two--mode cubic phase states. The
statistical properties of these states can be widely adjusted by tuning the
heterodyne mixing angles, the phases of the nonlinear couplings, as well as the
strength of the nonlinearity. For quadratic nonlinearity, we study the
higher-order contributions to the susceptibility in nonlinear media and we
suggest possible experimental realizations of multiphoton conversion processes
generating the cubic-phase heterodyne squeezed states.