Small-angle X-ray scattering and atomic force microscopy were used to observe smectic-smectic phase segregation in binary mixtures of rigid-rod-like helical poly[n-decyl-(S)-2-methylbutylsilane] (PDMS) polymers with molecular-weight ratios of 4.82 and narrow molecular-weight distributions. Phase segregation is attributed to entropic effects as the homopolymers in the binary mixture differ only in molecular weight. Entropy-driven segregation in smectic phases was theoretically predicted in mixtures of rodlike particles with different lengths under high pressure. Binary mixtures of long and short PDMS with broader molecular-weight distributions, which do not form smectic phases, showed no such segregation, verifying that the driving force for segregation is the entropy gained through smectic-phase formation. The binary mixture of a long PDMS with a narrow molecular-weight distribution and a short PDMS with a broad molecular-weight distribution showed segregation of each component, indicating that the entropy gain of short-polymer-only smectic phase formation surpasses the loss of mixing entropy.