Solution-processed plastic semiconductors have garnered significant attention recently due to their ease fabrication and diverse optoelectronic functionalities, positioning them as promising contenders for the next generation of semiconductors. However, comprehending the molecular ordering in polymer semiconductor blends during solution processing remains a captivating challenge. In this study, we chose poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(3-hexylselenophene-2,5-diyl) (P3HS) blends as the model system and examined the molecular ordering of blends with low molecular weights (below the entanglement) and high molecular weights (above the entanglement). By employing a combination of structural analysis, spectroscopic techniques, and theoretical modeling, valuable insight regarding the arrangement of molecules in three dimensions within P3HT/P3HS blends of varying molecular weights have been acquired. Through these analyses, we establish a comprehensive relationship between molecular weight, molecular ordering, and exciton coherence in polymer–polymer blends.