Measurements of open beauty and charm production cross sections in deep inelastic $ep$ scattering at HERA from the H1 and ZEUS Collaborations are combined. Reduced cross sections for beauty and charm production are obtained in the kinematic range of photon virtuality $2.5 \le Q^2 \le 2000$ GeV$^2$ and Bjorken scaling variable $3 \times 10^{-5} \le x_{\rm Bj} \le 5 \times 10^{-2}$.
The combination method accounts for the correlations of the statistical and systematic uncertainties among the different data sets. The combined data are compared to perturbative QCD predictions
and used together with the combined inclusive deep inelastic scattering cross sections from HERA to determine the charm and beauty quark masses.