Selective Coordination‐Driven Kinetic Synchronization of Two‐Step Tin‐Lead Perovskites for High‐Performance All‐Perovskite Tandem Solar Cells

All‐perovskite tandem solar cells hold great promise for high power conversion efficiencies. However, their narrow‐bandgap tin‐lead (Sn‐Pb) mixed perovskite subcells often suffer from severe crystallization imbalance under conventional one‐step solution processing, due to the intrinsically different behaviors of Sn‐ and Pb‐based precursors. Herein, we introduce a kinetic control strategy for Sn‐Pb perovskite crystallization via intermediate‐phase competition in an optimized two‐step sequential deposition process. Through systematic molecular screening, we identified 2‐amino‐2‐cyanoacetamide (ACA), a multidentate chelating molecule with distinct Lewis basic sites, as an effective intermediate‐phase regulator. ACA selectively coordinates with SnI 2 and PbI 2 via different functional groups, competing with the solvent to modulate precursor release and suppress unbalanced intermediates. This regulation in the first step forms a porous, preferentially oriented SnI 2 /PbI 2 framework and enables complete precursor conversion in the second step, yielding high‐quality Sn‐Pb perovskite films with minimal byproducts, reduced strains, and improved interfacial properties. Consequently, two‐step sequentially deposited single‐junction Sn‐Pb solar cells reach a PCE of 22.75%, while all‐perovskite tandem devices achieve 28.75%, accompanied by substantially enhanced operational stability, improved scalability, and good reproducibility. This strategy overcomes long‐standing Sn‐Pb crystallization imbalance and provides a versatile route to efficient all‐perovskite tandem photovoltaics.