Oy, I've had a frustrating day. Ran into a book that absolutely refused to download as AZW3 after converting to dual-mobi and Send-to-Kindle. I tried all the ideas suggested in this thread, including converting mobi>epub>mobi and using Kindlegen.
Also, if I simply copy the dual-mobi directly to my Kindle, it will open the AZW3 part. But there's something Amazon doesn't like about it so that when it goes through Send-To-Kindle the AZW3 part fails
