Board Thread:General Discussion/@comment-10674901-20130610071228/@comment-10630156-20130614130358

@Spyyke. White (the hollow that attacked masaki) was made from shinigami souls and Ichigo inherited part of this (hollow ichigo) which was awakened in the shattered shaft. "Zangetsu" was inherited from masaki because she was a pure blooded quincy. The way it is split is simple as this

White/Masaki's inner hollow (shinigami hollow hybrid) => Hollow Ichigo (Shinigami hollow hybrid) Masaki's quincy powers => "Zangetsu"/Ichigo's quincy powers

They weren't combined from the start as they came from two different powers within Masaki one planted by accident by Aizen and the other naturally occurring within her. As for the combining this was there way of trying to prevent Ichigo from learning FGT. I remember one saying "we are both part of your power" and as such could combine easily but as for them originally being the same being in the past this is highly unlikely