  Tunneling through two vertically coupled quantum dots is studied by means of
a Pauli master equation model. The observation of double peaks in the
current-voltage characteristic in a recent experiment is analyzed in terms of
the tunnel coupling between the quantum dots and the coupling to the contacts.
Different regimes for the emitter chemical potential indicating different peak
scenarios in the tunneling current are discussed in detail. We show by
comparison with a density matrix approach that the interplay of coherent and
incoherent effects in the stationary current can be fully described by this
approach.
