Mixed halide, mixed cation lead perovskite films have been demonstrated to benefit tremendously from the addition of Cs and Rb into the perovskite formulation, resulting in high performance, enhanced reproducibility and stability. However, the root cause of these effects in these complicated systems is not well understood. We address the above challenge by tracking in situ the solidification of perovskite precursors during solution-casting using time-resolved grazing incidence wide-angle X-ray scattering (GIWAXS). In doing so, we can directly link the formation or suppression of different crystalline phases to the presence of Cs and/or Rb. In the absence of these elements, the multi-component perovskite film is inherently unstable, phase segregating into a solvated MAI-rich phase and a FABr-rich phase. Adding even one of the two (Cs or Rb) is shown to alter the solidification quite dramatically, promoting different solidification pathways. Importantly, the addition of both components in the optimal ratio can drastically suppress phase segregation and promotes the spontaneous formation of the desired perovskite phase. This result is also confirmed by elemental mapping of organic cations (FA+, MA+) and halide anions (I-, Br-) via time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Perovskite precursors with an optimal combination of additives (7% Cs, 3% Rb) result in solar cells with 20.1% power conversion efficiency (PCE), outperforming formulation excluding Cs and Rb (PCE=14.6%). We propose that the synergistic effect is due to the collective benefits of Cs and Rb on the formation kinetics of the perovskite phase, and on the halides redistribution throughout the film. Importantly, our study points to new design rules for tuning the crystallization pathway of multi-component hybrid perovskites.