Actually, jetson does not put all the components on the NVMe. Even when you “boot from nvme”, there are still some components on the emmc. This is the point 1 in below thread.
You can totally boot from nvme after rel-32.6. Which means the kernel and file system are both on the NVMe drive. I see no problem with that. Why do you still want to put something on the emmc?
Both emmc and NVMe will be some device node like /dev/mmcblk0p1 and /dev/nvme0np1 after you enter kernel. You can mount them with common ubuntu commands (mount/umount).