This study systematically evaluated the agronomic viability of a novel double-cropping system through heterografting between tomato (Solanum lycopersicum L.) and potato (Solanum tuberosum L.). Using cleft grafting, two determinate tomato scions (Thilina and Padma) were grafted onto the vigorous potato rootstock Granola under protected cultivation in Sri Lanka's up-country region. Results demonstrated a critical scion-specific interaction governing system productivity. The Granola/Padma combination exhibited a greater compatibility, increasing tomato fruit yield by 2.8-fold and potato tuber yield by 2.1-fold per plant compared to non-grafted controls. Yield enhancement was exclusively mediated through a significant increase in the number of fruits and tubers, not their individual size. In contrast, the Granola/Thilina graft increased tomato yield but suppressed tuber initiation, highlighting a competitive interaction. Heterografting modified plant architecture, reducing height by 21–24% while maintaining or enhancing leaf area, and caused only minor, agronomically insignificant delays in phenology with no detriment to fruit quality. These findings confirm that interspecific grafting within Solanaceae is a functionally viable intensification strategy. The synergistic Granola/Padma graft establishes a scientifically validated model for dramatic land-use efficiency gains, leveraging complementary resource partitioning and source-sink balance to optimize productivity from a single plant, with direct applicability for smallholder systems.