By the use of quasi-elastic light scattering, dynamic critical behavior was studied for symmetrical blend solutions of polystyrene/poly(methyl methacrylate)/deuterated benzene and polystyrene/poly(2-chlorostyrene)/deuterated benzene having different polymeric indices N and polymer concentrations c. The decay rate Gamma(I) of polymer-composition fluctuations showed the dynamic crossover from the diffusive to the mode-mode-coupled hydrodynamic behavior in all solutions. The dynamic-crossover number tau*, which represents the distance between the stability limit and the location of the dynamic crossover, was comparable to that theoretically expected for the dynamically-unentangled system in the present ranges of N and c including a chain-entangled region. The blend solutions with entanglements exhibited a wider mode-mode-coupled regime than theoretical prediction. The dynamic-crossover number tau* was larger than the Ginzburg number Gi, which characterizes the location of the static crossover from the mean-field to the nonclassical Ising behavior, and the mode-mode,coupled dynamics covered a wider region than the statical Ising behavior.