The electro-catalytic oxidation of CO and CO/H-2 mixtures on polycrystalline Pt has been investigated on two electrodes with controllable degrees of coupling strength by means of experiments and numerical simulations. With strong positive coupling, large differences between the currents of the two almost-identical electrodes arise in a range of potential. Experiments on CO oxidation were carried out in three different electrolytes; all three electrolytes support pattern formation to an extent in the following order: 0.05 mol/L sulfuric acid with saturated sodium sulfate > 0.05 mol/L sulfuric acid > 0.1 mol/L perchloric acid. Experiments with 2% CO/ H-2 mixtures also exhibit nonuniform states with coupling. Numerical simulations on CO electro-oxidation confirmed the experimental findings: the current differences occur due to a symmetry breaking both with identical electrodes (through a pitchfork bifurcation) and with electrodes with inherent heterogeneities (due to a formation of an isola). The nonuniform reaction rates due to small inherent heterogeneities are greatly amplified by coupling. Simulations also show that the total coverage of CO and OH is always lower (and the number of total free active sites higher) in the nonuniform states; this suggests the possibility of increased total reaction rate not only in the CO oxidation system but also in systems in which the CO electro-oxidation occurs in parallel with other chemical and electrochemical reaction steps.