The jz4740 SoC present on the Dingoo A320 has a usb-boot mode (the famous "usb mode", when you press B while powering on the device). With this, you can always boot a linux kernel, even if the dingoo's system is trashed, and even without any SD card.
Booboo's dual-boot installer is just that, a linux kernel with integrated tools to flash the custom bootloader, that you load by the usb-boot mode; it has nothing to do with the native OS, and does work the same without any system on the NAND.