The separation of Co(II) and Ni(II) in aqueous two-phase systems (ATPSs) formed by the polymer polyethylene glycol 20,000 and several sodium electrolytes, coupled with the organic extractant 1-nitroso-2-naphthol was studied. First, the liquid-liquid equilibrium experiments were carried out to describe the phase diagrams of the ATPS. Then, the metal separation efficiency was evaluated considering several factors. It was observed that pH had a great influence on the selective metal chelation. When lowering the pH to 2 and 1 in the system with sodium sulfate, it was possible to suppress the Ni extraction under 5%. These results showed that the differences in the metal chelate stabilities could displace the equilibrium reaction with Ni(II) for selective extraction. In addition, experimental results showed that the interactions between metal cations and the salt anions also determined the separation efficiency. For the system with sodium sulfate (Na2SO4), Ni(II) extraction was suppressed at pH 2, but when sodium citrate (Na3C6H5O7) was used, the suppression occurred at pH lower than 0.5, while sodium tartrate (Na2C4H4O6) and sodium hydrogen phosphate (Na2HPO4) were not suitable for selective extraction. High separation factors of 11,080, 1136, and 3300 were obtained for the aqueous systems with Na2SO4 at pH 2, and pH 1, and Na3C6H5O7 at pH 0.5, respectively. Finally, the extraction at higher metal ion concentrations was tested. When the concentration of 1N2N was 1000 mg/kg, the maximum amount of extracted Co reached about 85 mg/kg within 15 min, which corresponds to complete extraction considering a metal:1N2N ratio of 1:4.