Simultaneous new drug application (NDA) submissions in China and Japan are increasingly important for global drug development, ensuring faster patient access and regulatory alignment. Multi-regional clinical trials (MRCTs) are essential for generating robust evidence tailored to diverse populations and meeting specific regional regulatory requirements. Achieving simultaneous regulatory submission across regions requires a clear, disciplined dossier development strategy from the beginning and continuous engagement and collaboration between global and local teams. Medical writers play a pivotal role in harmonising global and regional documentation, integrating messages, and driving operational efficiency through collaborative authoring practices. By addressing national requirements, such as China and Japan’s unique dossier expectations, sponsors can compress timelines, reduce redundancy, and enhance dossier quality for regulatory approval.