We probe the quantum response of plasmonic heterodimers with extended (plane waves) and localized (point dipoles and electron beams) sources. Implementing the surface-response formalism based on Feibelman parameters, we explore nanosphere heterodimers where one particle is characterized by electron spill-in, while the other by spill-out. We identify situations where the two quantum surface corrections that act in opposite spectral-shift directions can cancel out, producing mesoscopic spectra that do not differ from the predictions of the local-response approximation. This picture changes dramatically once localized sources are implemented, allowing one to selectively probe the quantum response of the particles independently. Such probing occurs in terms of higher-order modes in the case of electron beams, or for the complete modal landscape in the case of point dipole sources. This description offers a new insight into the mechanisms governing light–matter interactions in extreme plasmonic architectures, and opens new perspectives for the understanding and tailoring of plasmonic–quantum emitter architectures.