Shared cloud interactions unveil a candidate binary-system supernova pair with no known analogue.

IC 443 is one of the most extensively studied supernova remnants in the Galaxy, yet the surrounding region remains shrouded in mystery. Here we show that 16 years of Fermi-LAT observations uncover extended gigaelectronvolt gamma-ray emission from G189.6+3.3, a source long hidden in the shadow of the much brighter IC 443. The gamma-ray morphology is consistent with the X-ray shell detected by eROSITA, indicating an origin of the emission in the remnant. Spectral and spatial analyses identify distinct hadronic and leptonic gamma-ray components associated with regions containing and lacking molecular gas, respectively, revealing a clear spatial segregation of these emission mechanisms. In the northern shell boundary, hadronic gamma rays coincide with an Hα filament linked to the S249 ionized hydrogen cloud. Morphological studies, crushed-cloud modelling and ultraviolet observations of this region support gamma-ray production through proton re-acceleration in compressed post-shock gas. The shared-cloud interaction positions G189.6+3.3 at the same distance as IC 443, with a linear separation between the two remnants and a time delay in their explosion events, bolstering the hypothesis that these two remnants are part of a binary system, both resulting from separate supernova events.