The integration of thin 2D perovskite layers at the 3D PVK/HTL interface is known to improve the power conversion efficiency and stability of perovskite solar cells (PSCs). In this work, we report a novel approach consisting of mixing two alkyl-based cations that form 2D perovskites (propylammonium iodide (PAI) and octyl ammonium iodide (OAI)) to form mixed cation 2D halide perovskite layers. The solar cells containing the mixed cations demonstrate enhanced device stability retaining 95% of performance after 1000 h under maximum power point tracking, far exceeding PSC samples with single OAI, 88% and PAI, 80% 2D layers. By cation mixing, the formation of a novel 2D perovskite crystal structure was confirmed by XRD and PL analysis and it was shown to have a uniform quasi-2D state (n = 2), a feature not found in single OAI and PAI samples. Notably, the novel morphology of the crystal structure leads to improved carrier dynamics and enhanced PSC stability.