We report on the segmental dynamics of the binary polymer blend polystyrene (PS)/poly(methylphenylsiloxane) (PMPS) in the two-phase region using dielectric spectroscopy that essentially probes the PMPS component. Based on the experimental orientation relaxation functions, the average glass transition temperature T-g controls phase separation. When the spinodal temperature T-s exceeds T-g, the PMPS segmental relaxation displays two distinct decays characteristic of a merely pure and a mixed, roughly at the initial composition, PMPS regions. On the contrary, when T-s falls in the proximity of T-g, the PMPS relaxation is strongly nonexponential and its average time reflects mixed regions rich in PMPS due to incomplete phase separation, which drives only the glassy phase out of local thermodynamic equilibrium. Distinct morphological differences in the two-phase state of these blends, inferred from their segmental dynamics, are revealed by transmission electron microscopy.