Actually, true double albums are counted twice -
Multi-disc albums are counted once for each disc within the album if it is over 100 minutes in length or is from the vinyl era. For example, each copy of The Smashing Pumpkins's Mellon Collie and the Infinite Sadness (running time of 121:39), OutKast's Speakerboxxx/The Love Below (running time of 134:56), and Shania Twain's Up! (145:44), all double albums, were counted twice, meaning each album was certified diamond after 5 million copies were shipped
BUT - Here is my thought. A double album is dangerous for U2. They are at a point now where they will have a couple of weeks of big sales from their die-hard fans. Maybe a few hundred thousand. But remember that double albums also cost more. So beyond the die-hard fans, you are gonna lose a lot of casual fans, and new fans that aren't gonna shell out 18 bucks for an album, but they may shell out 8 to 10 bucks much more readily. More likely they would download the one big single if they manage to get one off this album.