I reckon the Master Xehanort (baldy) and the split heartless were the same person, and are the heartless and nobody versions of him. And somehow they have found each other and as a result it shows the immense power they have working simultaneously, I mean they beat the crap out of the Keyblade warriors.
The flaw with your original idea is that she may be a Princess of Heart but she wasn't a princess of Radiant Garden. In the Real Ansem's notes, Ansem mentions Kairi as a "resident of Radiant Garden".
