You guys are arguing over something that is just false as of the moment. We don't know if BHK has half of Soras heart....And I doubt that it could happen. He isn't his brother...They hardly look like brothers...Sora has the sort of American face build and BHK has the Asian face build...Unless they were seperated at birth, I don't think it will happen...Or could happen. BHK isn't Sora's shell because in order to use a Keyblade, you have to be able to have a heart...period.
I simply think that BHK is Soras other half of his heart. In CoM, Vexen didn't want Sora to reach the other side because then he would find out everything he would need to know. Now what I mean by other half is not literally BHK has the other half...I think they are like the same person, just living in different existences. Like, BHK is his own person...But like the Sora of the In-Between world. When Sora went to Twilight Town, he said it was familiar to him, yet he didn't know exactly what it was. I'm guessing that if Kairi went there, she would feel the same thing..Sort of like a him just in another realm...Begruding both Light and Dark..
In conclusion, I just think that BHK and Sora are both individual people...But just seem to be connected by the fact that BHK is like the Sora of the In-Between world.