We studied the interfacial tension between coexisting phases of aqueous solutions of dextran and polyethylene glycol. First, we characterized the phase diagram of the system and located the binodal. Second, the tie lines between the coexisting phases were determined using a method that only requires measuring the density of the coexisting phases. The interfacial tension was then measured by a spinning drop tensiometer over a broad range of polymer concentrations close to and above the critical point. In this range, the interfacial tension increases by 4 orders of magnitude with increasing polymer concentration. The scaling exponents of the interfacial tension, the correlation length, and order parameters were evaluated and showed a crossover behavior depending on the distance to the critical concentration. The scaling exponent of the interfacial tension attains the value 1.50 ± 0.01 further away from the critical point, in good agreement with mean field theory, but the increased value 1.67 ± 0.10 closer to this point, which disagrees with the Ising value 1.26. We discuss possible reasons for this discrepancy. The composition and density differences between the two coexisting phases, which may be taken as two possible order parameters, showed the expected crossover from mean field behavior to Ising model behavior as the critical point is approached. The crossover behavior of aqueous two-phase polymer solutions with increasing concentration is similar to that of polymer solutions undergoing phase separation induced by lowering the temperature.