Viscoelastic, dielectric, and rheo-optical behavior was examined fir miscible blends of high-M cis-polyisoprene (PI) and poly(p-tert-butyl-styrene) (PtBS). The slow dielectric relaxation of the blends was exclusively attributed to the global motion of the PI chains having the type-A dipoles. The PI and PtBS chains behaved as the fast and slow (low- and high-friction) components and were well entangled with each other. The dynamics of these chains changed significantly with temperature T. At high T, the blend exhibited two-step entanglement plateau of the storage modulus G' (omega), and the plateaus at high and low angular frequencies (omega) were attributed, with the aid of the dielectric data, to the entanglement among all component chains and that between the PtBS chains, respectively. The entanglement length a characterizing the high-omega plateau was well described by a simple mixing rule based on the number fraction n of the Kuhn segments of the components, a = n(PI)a(PI)(bulk) + n(PtBS)a(PtBS)(bulk). This result was consistent with the current molecular picture relating the entanglement density to the packing length p (congruent to a/20). The complex moduli G* of the blends in the high-omega plateau zone were well described by a simple blending law combined with this mixing rule of a, which was consistent with the rheo-optical data. At low T, the blend exhibited the Rouse-like power-law behavior of storage and loss moduli, G' = G '' proportional to omega(1/2), in the range of omega where the high-omega plateau was supposed to emerge. This lack of tie high-omega plateau was attributed to retardation of the Rouse equilibration of the PI chain over the entanglement length a due to the hindrance from the slow PtBS chains: The PI and PtBS chains appeared to be equilibrated cooperatively/simultaneously at a rate essentially determined by PtBS. The Rouse equilibration time, evaluated from the G* data of the blend, was just moderately shorter than the dielectrically determined relaxation time of PI. Thus, the high-omega plateau zone was too narrow to be resolved experimentally, and the PI chains relaxed almost immediately after their Rouse equilibration (retarded by PtBS). This PI relaxation activated the constraint release (CR) relaxation of PtBS to dilate the entanglement mesh for TtBS. A simple model considering the Rouse equilibration and CR/dilation processes described the G* data of the blend surprisingly well, lending support to the molecular picture of the cooperative/simultaneous Rouse equilibration of the PI and PtBS chains. The model calculation was consistent with the rheo-optical data, which lent further support to this molecular picture.