Choosing to boot from eMMC or SD card using debug port

I want to install an additional root file system on the SD card and use the debug port to choose from either eMMC or the SD card. How to prepare the SD card to achieve this? And what changes should I do to the u-boot to achieve this? Basically, I want to have a dual boot.

Things I tried
I split the SD card into two partitions. I copied the rootfs to the first partition (ext4) and executed

./ -r /media/SDcard/rootfs

Below is the dump of the u-boot printouts on the debug port.

U-Boot 2016.07 (Feb 28 2020 - 11:42:29 +1100)

Model: NVIDIA P2771-0000-500
DRAM:  7.8 GiB
MMC:   Tegra SD/MMC: 0, Tegra SD/MMC: 1
*** Warning - bad CRC, using default environment

In:    serial
Out:   serial
Err:   serial
Net:   eth0: ethernet@2490000
Hit any key to stop autoboot:  0 
switch to partitions #0, OK
mmc1 is current device
Scanning mmc 1:1...
switch to partitions #0, OK
mmc0(part 0) is current device
Scanning mmc 0:1...
Found /boot/extlinux/extlinux.conf
Retrieving file: /boot/extlinux/extlinux.conf
733 bytes read in 68 ms (9.8 KiB/s)
L4T boot options
1:	primary kernel
Enter choice:

I am also a bit confused about the role the extlinux.conf file in the booting of the kernel. It just helps us choose between kernel images on the same partition, right?

Hi vinmeen,

Please reference Topic-1060950 steps to flash on external devices.
We are not support multiple boot device, you only can boot from emmc or external devices.

I found the topic in the forum which hints that it could be implemented.

I understand that such a dual boot is not supported by Nvidia but can some one help me with general idea/procedure which could achieve dual boot?

or is the ‘dual boot’ not possible at all? Please help me understand