Well in the Secret Ending Terra says "You can take my Body but my Hearts is mine" and then MX finds out Eraqus' Heart is also in Terra and if MX puts his heart in Terra then Xehanort = Terra's Heart & Body + MX's Heart + Eraqus Heart. So when Xehanort fell to darkness we should have 4 Beings. Terra's Nobody, MX's Heartless, Terra's Heartless, and Eraqus Heartless. We've only seen two, Terra's Nobody (Xemnas) and A Heartless named Ansem SOD (Most likely XM's Heartless; Could be Terra or Eraqus). So where are the other 2 Heartless?