Basically, the boot loader will load the kernel and the kernel will mount initfs and run some setup stuff, then mount the root fs and launch init. What people do to "dual boot" is have the linux kernel run a script in initfs that lets the user pick which rootfs to mount from a menu. That allows them to have the rootfs on an sd card. This is a great way to test out new things and always leave your device in a bootable state. Ask more questions if you have them.

Also check out the deblet debian-on-n8x0, at least for ideas on how people got other distros booting. Have fun and tell us how it goes!

-John