You'll notice in the main menu where you change the difficulty in Unova Link, there's an option with different keys. By activating the respective keys you can make the Regis appear in the Underground Ruins, which is within Clay Tunnel.
Registeel requires the Iron Key, which is exclusive to Black 2.
Regice requires the Iceberg Key, which is exclusive to White 2.
I believe Regirock is in both versions.
However, like you're able to exchange difficulties via Unova Link, the keys can be exchanged between versions.