More and more observations show that large ground-level
enhancement (GLE) events are often associated with twin coronal mass
ejections (CMEs). In the twin-shock scenario, the preceding
CME-driven shock provides an enhanced-level turbulence at the
upcoming CME-driven shock. In this work, we focus on the GLE event
on 2012 May 17, which would be associated with the twin-shock
interactions. Using the particle simulation method to examine the
effect of the preceding CMEs being surrounded by the forthcoming
CMEs on the energy spectrum. We use the preceding CMEs-driven shock
to produce an enhanced seed population injecting into the later
CME-driven shock for particle re-acceleration. As results, we
simulate the twin-shock interactions and obtain the compound proton
spectrum. Therefore we suggest that the twin-shock interactions
would produce an GLE event with an ``ankle-like'' proton spectral
slope.