This paper applies geometric methods developed to understand chaos and transport in Hamiltonian systems to the study of power distribution in nonlinear waveguide arrays. The specific case of two linearly coupled ͑2͒ waveguides is modeled and analyzed in terms of transport and geometry in the phase space. This gives us a transport problem in the phase space resulting from the coupling of the two Hamiltonian systems for each waveguide. In particular, the effect of the presence of partial and complete barriers in the phase space on the transfer of intensity between the waveguides is studied, given a specific input and range of material properties. We show how these barriers break down as the coupling between the waveguides is increased and what the role of resonances in the phase space has in this. We also show how an increase in the coupling can lead to chaos and global transport and what effect this has on the intensity. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2840461͔We first present the general physical model for an arbitrary number of quadratic nonlinear waveguides, coupled linearly to each other. We normalize the system so that the dynamical evolution variable, i.e., the distance along the waveguides, can be big and therefore considered as a time variable, as is conventional in transport and chaos studies. We then focus on the two-core coupler (two waveguides) and investigate how structures in the phase space can lead to information about the coupling required for complete transfer of intensity from the fundamental wave (FW) in one waveguide to the second harmonic (SH; at twice the frequency) in the other waveguide. In particular, we investigate the structures that act as partial or complete barriers in the phase space. We study how these structures are destroyed as the coupling between the two waveguides is increased from the uncoupled regime to a critical value and beyond. The present work also includes a study of the effect of resonances in the phase space, as well as their connection with the destruction of complete barriers to transport. The sensitivity of the resulting intensity distribution to the strength of the coupling between the FW in the two waveguides "… is also studied. In particular, we show how the nature of the structures (or barriers) in the phase space can be changed with small changes in , such that a very ordered output can change to an output which appears ordered over a long time, but then suddenly becomes disordered or more complicated, to finally reach a very disordered state appearing to be the product of spatial-temporal chaos. Such behavior is of much practical importance, as it shows that, for certain values of , a small change in can produce very different outputs. This also shows that such sensitivity is inherent in the system and is not a product of noise or experimental error.