  We compute the in-medium energy loss probability distribution of two
neighboring subjets at leading order, in the large-$N_c$ approximation. Our
result exhibits a gradual onset of color decoherence of the system and accounts
for two expected limiting cases. When the angular separation is smaller than
the characteristic angle for medium-induced radiation, the two-pronged
substructure lose energy coherently as a single color charge, namely that of
the parent parton. At large angular separation the two subjets lose energy
independently. Our result is a first step towards quantifying effects of energy
loss as a result of the fluctuation of the multi-parton jet substructure and
therefore goes beyond the standard approach to jet quenching based on single
parton energy loss. We briefly discuss applications to jet observables in
heavy-ion collisions.
