It's actually quite difficult to support a dual boot Raspbian... I've tried it before but think it's not the right answer, in fact the right answer would be to put the ext4 partition on the hard drive and just the FAT filesystem on the eMMC
Unfortunately the NOOBS recovery system has not been extended far enough for this to be possible and I really don't have the time to do this.
I was thinking of being able to create and support a Raspbian image as well (i.e. including the LED drivers and codec driver) and sell that separately that way you could switch between two instead.