I’ve think I’ve done essentially what you’re proposing (though on ARM) where my NOR storage contains:
Bootloader
RootfsA - fitImage containing kernel, device tree and squashfs initramfs
RootfsB - fitImage containing kernel, device tree and squashfs initramfs
Data
It works well but there were some patches required to both the Mender client code and the Mender uboot code to facilitate loading fitImages and having a rootfs mounted in RAM. There’s some info in some of my previous posts: Read only compressed rootfs in raw flash?, Updating when rootfs is in RAM? and Xilinx Zynq - change mender to have rootfs inside fit image.
As Drew said, it might be easier to use update modules but thought it was worth sharing in case it helps!