Is it possible to have a fallback file system on eMMC, while having the main one on an NVMe?

Hi,

I am trying to have a fallback rootfs on the eMMC, the main file system is on an NVMe. I am using JetPack-5.0.2 and an Xavier AGX. I have tried setting up a backup entry in extlinux.conf with the only difference being the root flag pointing to the partition where the other file system lives (in this case, partition 1 of the eMMC), like so:

TIMEOUT 30
DEFAULT primary

MENU TITLE L4T boot options

LABEL primary
MENU LABEL primary kernel
LINUX /boot/Image
FDT /boot/dtb/kernel_tegra194-p2888-0001-p2822-0000.dtb
INITRD /boot/initrd
APPEND ${cbootargs} root=/dev/nvme0n1p1 rw rootwait rootfstype=ext4 mminit_loglevel=4 console=ttyTCU0,115200n8 console=tty0 fbcon=map:0 net.ifnames=0 rootfstype=ext4 sdhci_tegra.en_boot_part_access=1

LABEL backup
MENU LABEL backup kernel
LINUX /boot/Image
FDT /boot/dtb/kernel_tegra194-p2888-0001-p2822-0000.dtb
INITRD /boot/initrd
APPEND ${cbootargs} root=/dev/mmcblk0p1 rw rootwait rootfstype=ext4 mminit_loglevel=4 console=ttyTCU0,115200n8 console=tty0 fbcon=map:0 net.ifnames=0 rootfstype=ext4 sdhci_tegra.en_boot_part_access=1

This does not seem to work. If there is no valid file system on the NVMe I get a kernel panic. I can manually choose to boot from the second entry using the UEFI GUI though, but I need this to happen automatically.

Questions:
1. Is there a better way to achieve having a file system and fallback file system on different storage devices?
2. Does A/B file system redundancy support having the fallback file system on another storage device?

Thanks,

Marisol