This work explores electrochemical impedance spectroscopy to study recombination and ionic processes in all-perovskite tandem solar cells. We exploit selective excitation of each sub-cell to enhance or suppress the impedance signal from each sub-cell, allowing study of individual tandem sub-cells. We use this selective excitation methodology to show that the recombination resistance and ionic time constants of the wide gap sub-cell are increased with passivation. Furthermore, we investigate sub-cell-dependent degradation during maximum power point tracking and find an increase in recombination resistance and a decrease in capacitance for both sub-cells. Complementary optical and external quantum efficiency measurements indicate that the main driver for performance loss is the reduced capacity of the recombination layer to facilitate recombination due to the formation of a charge extraction barrier. This methodology highlights electrochemical impedance spectroscopy as a powerful tool to provide critical feedback to unlock the full potential of perovskite tandem solar cells.