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I think Kyurem being weaker than Zekrom and Reshiram is the key to understanding why it cannot properly house both of them at the same time in its fusion in BW2. Kyurem isn't just a shell of the dragon, it is a degraded shell. It would be like the difference between Wuji and Taiji. Kyurem needs a better more pure form if it going to be able to accept both yin and yang.
