PSD-95 is a scaffolding protein of the MAGUK protein family, and engages in several vital protein-protein interactions in the brain with its PDZ domains. It has been suggested that PSD-95 is composed of two supramodules, one of which is the PDZ1-2 tandem domain. Here we have developed rigidified high-affinity dimeric ligands that target the PDZ1-2 supramodule, and established the biophysical parameters of the dynamic PDZ1-2/ligand interactions. By employing ITC, protein NMR, and stopped-flow kinetics this study provides a detailed insight into the overall conformational energetics of the interaction between dimeric ligands and tandem PDZ domains. Our findings expand our understanding of the dynamics of PSD-95 with potential relevance to its biological role in interacting with multivalent receptor complexes and development of novel drugs.