Question

Where do you find the keys to the Regi chambers in B/W2?

Answer 1

You need both W2 and B2 to get the keys needed; Registeel's Iron Key is obtained in Black 2 and Regice's Iceberg Key is obtained in White 2. You get these keys in your version when you capture Regirock.

You then swap keys via the Unova Link function between the two versions.

Source

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If you're asking how to solve the puzzle, watch this video:

Link