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The discovery of emergent phenomena in 2D materials has sparked substantial research efforts in the materials community. A significant experimental challenge for this field is exerting atomistic control over the structure and composition of the constituent 2D layers and understanding how the interactions between layers drive both structure and properties. While no segregation for single bilayers was observed, segregation of Pb to the surface of three bilayer thick PbSe–SnSe alloy layers was discovered within [(Pb_xSn_1−xSe)_1+δ]_n(TiSe₂)₁heterostructures using electron microscopy. This segregation is thermodynamically favored to occur when Pb_xSn_1−xSe layers are interdigitated with TiSe₂monolayers. DFT calculations indicate that the observed segregation depends on what is adjacent to the Pb_xSn_1−xSe layers. The interplay between interface‐ and volume‐free energies controls both the structure and composition of the constituent layers, which can be tuned using layer thickness.