A study of coupling characteristics between two optically nonlinear planar waveguides has been performed in terms of their individual analytic solutions. It is shown that with an appropriate choice of guide separation and proper redefinition of effective propagation constants, the commonly adopted dual waveguide coupled equations can be formally retained even when the optical nonlinearity in each guide is fully taken into account. The coupling coefficients are determined explicitly in terms of the individual mode fields and the associated intensity-dependent effective index of refraction. A specific numeric illustration of the power flow pattern was given on the basis of its analytical expression derived from the coupled equations. The result describes the detailed coupling characteristics and its variation with respect to input optical power, demonstrating its viability for active optical device applications.