We report the development of a new technology for monitoring multidimensional protein-protein interactions (PPIs) inside live mammalian cells using split RNA polymerase (RNAP) tags. In this new system, a protein-of-interest is tagged with an N-terminal split RNAP (RNAP(N)), and multiple potential binding partners are each fused to orthogonal C-terminal RNAPs (RNAP(C)). Assembly of RNAP(N) with each RNAP(C) is highly dependent on interactions between the tagged proteins. Each PPI-mediated RNAP(N)-RNAP(C) assembly transcribes from a separate promoter on a supplied DNA substrate, thereby generating a unique RNA output signal for each PPI. We develop and validate this new approach in the context of the Bcl-2 family of proteins. These key regulators of apoptosis are important cancer mediators, but are challenging to therapeutically target due to imperfect selectivity that leads to either off-target toxicity or tumor resistance. We demonstrate binary (1 x 1) and ternary (1 x 2) Bcl-2 PPI analyses by imaging fluorescent protein translation from mRNA outputs. Next, we perform a 1 x 4 PPI network analysis by direct measurement of four unique RNA signals via RT-qPCR. Finally, we use these new tools to monitor pharmacological engagement of Bcl-2 protein inhibitors, and uncover inhibitor-dependent competitive PPIs. The split RNAP tags improve upon other protein fragment complementation (PFC) approaches by offering both multidimensionality and sensitive detection using nucleic acid amplification and analysis techniques. Furthermore, this technology opens new opportunities for synthetic biology applications due to the versatility of RNA outputs for cellular engineering applications.