  Third-order nonlinear processes require phase matching between the
interacting fields to achieve high efficiencies. Typically in guided-wave
$\chi^{(3)}$ platforms this is achieved by engineering the dispersion of the
modes through the transverse profile of the device. However, this limits the
flexibility of the phase matching that can be achieved. Instead, we analyze
four-wave mixing in a pair of asymmetric waveguides and show that phasematching
may be achieved in any $\chi^{(3)}$ waveguide by coupling of a nondegenerate
pump from an adjacent waveguide. We demonstrate the additional flexibility that
this approach yields in the case of photon-pair generation by spontaneous FWM,
where the supermode dispersion may be modified to produce pure heralded single
photons -- a critical capability required for example by silicon platforms for
chip-scale quantum photonics.
