Originally Posted by Doitsu
However, there's no guarantee that the combined file will be accepted by KDP.
Also there's a distinct possibility that fixed layout mobis will be made available only to Kindle Fire users and Kindle apps with KF8 support. I.e. the non-KF8 mobi part might be discarded anyway.
When published on Amazon fixed-layout books can currently only be downloaded to Kindle Fire and Kindle for Android. Even Kindle for PC/Mac isn't an option, though it does support fixed-layout books as of the most recent revision.
If you're just producing books for your personal enjoyment, or at least distribution through outlets other than Amazon, the "master mobi" files created by KindleGen will load properly on devices and apps supporting either Kindle Format 8 or the older MobiPocket format. However, if you're intending to sell the books through Amazon there's really no point in even worrying about the legacy readers since customers won't be able to download the books to those devices.