Xemnas is also a special case, yes, but in an other way then Naminé and Roxas.
While Naminé and Roxas developed their own senses of self
independent from their true counterparts due to Kairi/Naminé and Sora/Roxas coexisting, is the case of Xemnas a tad more complicated.
Like Alex999 said, Xemnas was created from a made-up being (Apprentice Xehanort) who had 3 hearts within him. The questions that remain are what gained Xemnas from which heart and in what amount.
This however is one of the riddles rumored to be addressed in KH 3D, how the powers/memories and and hearts from Master Xehanort, Terra and Master Eraqus were divided between Ansem, Seeker of Darkness and Xemnas.
what id like to know though is what defines the shape the nobody takes after if hes made from someone thats carrying more than one heart. probly not relevant, still curious
The shape is probably defined by the heart that is leftover, not by the one that got overtaken by darkness and became a heartless.
Best Example is again Roxas, Sora's heart was overtaken by darkness, expelling Ventus's injured one in the process which remained in the body and shaped it to look like its original body, Ventus. Ergo Roxas has Ventus's looks.
For Xemnas, as already said, MX/Terra's and Eraqus's hearts are implied to be meshed together differently than Sora's and Ven's, not to mention the former connection was a forced one compared to the latter, so we don't know it exactly, but judging by Xemnas's looks it seems he was influenced by MX's heart and Terra's as well.