Ok, I was wondering about this and I thought I cam up with a small theory. BHK is able to dual-weild keyblades, we've all seen it. But whenever he is seen with two, he doesn't seem to be in a fused state(unless he's always in one). Where as Sora has to fuse to obtain two(two hearts, two keyblades). At first I thought, maybe BHK has another heart within him. But that wouldn't be possible, because that would mean throughout KH, Sora could've used two keyblades. But then I remembered how Riku temporarily can't use a keyblade because he still has too much darkness within him. So a thought came to me. That would mean there are 3 keyblade weilders who can use the keyblade. But if there is a keyblade for each dimension(which seems to be a popular idea) then that would mean there would be one that wasn't in use. So i thought, since Riku can't use the keyblade, maybe it decided to reside with BHK. That would mean whenever someone was out of action in a way of speaking, that another weilder would be able to use their keyblade. Please come up with your own opinions on this and you can criticize me as much as you want. I can see a few things that make this faulty, but it seems like a good solid idea, at least to me.