  In this work, a quantum dot couples to two helical edge states of a 2D
topological insulator through weak tunnelings is studied. We show that if the
electron interactions on the edge states are repulsive, with Luttinger liquid
parameter $ K < 1 $, the system flows to a stable two-channel fixed point at
low temperatures. This is in contrast to the case of a quantum dot couples to
two Luttinger liquid leads. In the latter case, a strong electron-electron
repulsion is needed, with $ K<1/2 $, to reach the two-channel fixed point. This
two-channel fixed point is described by a boundary Sine-Gordon Hamiltonian with
a $K$ dependent boundary term. The impurity entropy at zero temperature is
shown to be $ \ln\sqrt{2K} $. The impurity specific heat is $C \propto
T^{\frac{2}{K}-2}$ when $ 2/3 < K < 1 $, and $ C \propto T$ when $ K<2/3$. We
also show that the linear conductance across the two helical edges has
non-trivial temperature dependence as a result of the renormalization group
flow.
