About Black and White (2)...

So, the game tells us that Reshiram and Zekrom were once one, and when split apart the empty shell which was Kyurem was left behind.

Then by BW2, we can fuse the shell Kyurem with 1 of the 2 legendaries. So I'm left thinking of why I can't fuse all 3. Am I wrong? Did I pick something up wrong along the way? If not, why would they just drop us with the ability to fuse the shell with but 1 and just leave it at that with no explanation?

Also, it is believed Kyurem came to Earth on a comet that crashed in the Giant Chasm. That Kyurem would go out at night and eat people so Lacunosa town built a wall. How does that add up with the other myth that the Kyurem was owned by the king in the form of Reshiram+Zekrom+Kyurem?

Maybe the Kyurem crashed in the form fused with the 2 legendary dragons, tormented the town until they built a wall, then the king captured him and they owned Kyurem for generations until the two princes forced him to seperate?

also Pokémon myth specualtion thread I guess