  Based on the knowledge of the QCD radiation pattern, observables to
distinguish jets containing one and two $b$-hadrons are discussed. A simple
method is used to combine pairs of the most sensitive observables, girth,
number of charged tracks and the energy or momentum fraction of the leading
$b$-hadron with respect to the jet, into one discriminator. Their efficiencies,
on particle level, are estimated and found to improve the performance and the
robustness of the observables in different momentum slices.
