After a really brief cursory review of the literature, I think that a Dalitz decay is a meson decay that involves two leptons in the final state, plus a photon. A double Dalitz decay has four leptons in the final state: see this paper and this paper for examples of the usage. The Dalitz decay is when a virtual photon from 2 photon decay of $\pi_0$ internally converts to a real lepton pair before it gets too far, and analogous thing for other meson or Higgs processes (two electrons from an internal photon conversion, plus a neutral object).
I guess that the usage comes from the kinematic decay product phase space is described by a Dalitz plot, hence the name. I don't think it's anything deep.