This article examines fragmentation between international trade law and international investment law, two closely related regimes of international economic law that share common post-World War II foundations but have developed along different legal and institutional paths. It focuses on the legal and economic implications of this divide, particularly where trade measures also affect protected investments. Using doctrinal analysis, case studies and insights from law and economics, the paper evaluates how fragmentation affects predictability, transaction costs, strategic behaviour and regulatory autonomy. The Philip Morris plain packaging and Ontario renewable energy disputes show that the same state measure may be assessed through different legal standards and remedial logics. The article argues that the problem is not fragmentation as such, but unmanaged or strategically exploitable divergence. It concludes that coherence should be pursued through selective coordination rather than institutional merger, including clearer treaty design, institutional dialogue, cross-regime awareness and reason-giving practices.