  We consider a system of two spins under a scanning tunneling microscope bias
and derive its master equation. We find that the tunneling elements to the
electronic contacts (tip and substrate) generate an exchange interaction
between the spins, as well as a Dzyaloshinskii-Moriya interaction in the
presence of spin-orbit coupling. The tunnel current spectrum then shows
additional lines compared to conventional spin resonance experiments. When the
spins have degenerate Larmor frequencies and equal tunneling amplitudes
(without spin-orbit), there is a dark state with vanishing decay rate. The
coupling to the electronic environment generates significant spin-spin
entanglement via the dark state, even if the initial state is non-entangled.
