  We report the neutrino mass hierarchy (MH) sensitivity of medium baseline
reactor neutrino experiments with multiple detectors. Sensitivity of
determining the MH can be significantly improved by adding a near detector and
combining both the near and far detectors. The size of the sensitivity
improvement is related to accuracy of the individual mass-splitting
measurements and requires strict control on the relative energy scale
uncertainty of the near and far detectors. We study the impact of both baseline
and target mass of the near detector on the combined sensitivity. A
figure-of-merit is defined to optimize the baseline and target mass of the near
detector and the optimal selections are $\sim$13~km and $\sim$4~kton
respectively for a far detector with the 20~kton target mass and 52.5~km
baseline. As typical examples of future medium baseline reactor neutrino
experiments, the optimal location and target mass of the near detector are
selected for JUNO and RENO-50. Finally, we discuss distinct effects of the
neutrino spectrum uncertainty for setups of a single detector and double
detectors, which indicate that the spectrum uncertainty can be well constrained
in the presence of the near detector.
