It's working like it's supposed to... both the MOBI6 version and the KF8 version are contained in one file (two MOBI headers). Just like kindlegen produces. A device/app capable of reading KF8 will read the KF8 portion and a device that's incapable will read the MOBI portion. If you wanted to make two separate stand-alone files... just forget the combined_format tweak and convert twice: once to MOBI and once to AZW3.
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