The authors report on the first successful two-electron coincidence experiment at solid surfaces in a backreflection geometry. They found that two-electron coincident electron emission events exist, and that they require a certain threshold energy of the primary electron (relative to the Fermi level) which is not equal to twice the work function of the sample surface (W(100)) as one might expect from the energy conservation law. They give evidence that they observe at low primary energy single electron-electron scattering events between the primary electron and a valence-band electron and give a semiquantitative estimate of the relative probability of such events.