  Since the experimental realization of Kondo physics in quantum dots, its
far-from-equilibrium properties have generated considerable theoretical
interest. This is due to the interesting interplay of non-equilibrium physics
and correlation effects in this model, which has by now been analyzed using
several new theoretical methods that generalize renormalization techniques to
non-equilibrium situations. While very good agreement between these methods has
been found for the spin-1/2 Kondo model, it is desirable to have a better
understanding of their applicability for more complicated impurity models. In
this paper the differences and commons between two such approaches, namely the
flow equation method out of equilibrium and the frequency-dependent poor man's
scaling approach are presented for the non-equilibrium double quantum dot
system. This will turn out to be a particularly suitable testing ground while
being experimentally interesting in its own right. An outlook is given on the
quantum critical behavior of the double quantum dot system and its
accessibility with the two methods.
