Solid-State Transformers with Input-Series/Output-Parallel configuration offer a convenient solution for AC/DC conversion due to their scalability and modularity. In this configuration, each module experiences a second-order harmonic ripple caused by local single-phase AC/DC conversion. To neutralize this ripple, Active Power Filters can be installed, eliminating the need for oversized DC-bus capacitances. However, the presence of multiple APFs, working at the same time, can lead to dynamic interactions and potential instability due to the coupling between different ISOP SST modules. This study examines the mutual dynamics arising from multiple APFs in ISOP SSTs. It is shown that ensuring stability for the operation of a single APF does not automatically guarantee the overall stability when all APFs are simultaneously enabled. To study this phenomenon, an analysis approach based on the Generalized Nyquist Criterion for Multi-Input/Multi-Output systems is derived. Through the proposed approach, the closed-loop stability only needs to be verified towards two equivalent SST impedances that intrinsically considers the mutual coupling. This provides a simple design and verification tool for the APF controller, and the results are validated experimentally using a single-phase ISOP SST.