  We report a dual resonance feature in ballistic conductance through a quantum
Hall graphene nanoribbon with a magnetic quantum dot. Such a magnetic quantum
dot localizes Dirac fermions exhibiting anisotropic eigenenergy spectra with
broken time-reversal symmetry. Interplay between the localized states and
quantum Hall edge states is found to be two-fold, showing Breit-Wigner and Fano
resonances, which is reminiscent of a double quantum dot system. By fitting the
numerical results with the Fano-Breit-Wigner lineshape from the double quantum
dot model, we demonstrate that the two-fold resonance is due to the valley
mixing that comes from the coupling of the magnetic quantum dot with quantum
Hall edge channels; an effective double quantum dot system emerges from a
single magnetic quantum dot in virtue of the valley degree of freedom. It is
further confirmed that the coupling is weaker for the Fano resonance and
stronger for the Breit-Wigner resonace.
