  We propose a new observational test of top-down source models for the
ultra-high-energy cosmic-rays (UHECRs), based on the simultaneous observation
of two or more photons from the same Galactic hadron jet. We derive a general
formula allowing one to calculate the probability of detecting such `multiple
events', for any particular top-down model, once the physical parameters of the
associated hadron jets are known. We then apply our results to a generic
top-down model involving the decay of a supermassive particle, and show that
under reasonable assumptions the next-generation UHECR detectors would be able
to detect multiple events on a timescale of a few years, depending on the mass
of the top-down progenitor. Either the observation or the non-observation of
such events will provide constraints on the UHECR top-down models and/or the
physics of hadronization at ultra-high energy.
