I'm working on compiling info for the OP and just realized something.
The RIAA counts sales of double-disc albums as two units toward certification (ie: Shania Twain's "Up" is a double disc album, has sold 5 million copies in the US and is certified diamond) so technically, by these rules is Hannah Montana 2: Meet Miley Cyrus eligible for 6x platinum certification?
It's a double disc album and it's sold upwards of 3 million copies so I don't see why it wouldn't...
I'm not sure if this is a dumb question or not so I'm asking it in here instead of making a thread.