Demands for single-photon sources are ubiquitous in quantum information processing as well as in quantum metrology. In many protocols for producing single photons, a cavity-emitter configuration is used. In such cavity quantum electrodynamical systems, the cavity can enforce a well-defined output mode for the photon and enhance its collection efficiency, while the emitter is indispensable for single photon emission. Here we show the two cavity-one two-level emitter configuration can be used to produce exclusively photon pairs, with each photon in a separate mode. Conditioning on detecting a photon in one of the modes, one heralds with high fidelity a single photon in the other mode. Counterintuitively, upon decreasing the coupling of the emitter to one of the modes, the heralding fidelity can further increase.
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