I have installed VIM_Ubuntu-mate-16.04_Linux-4.9_V170831 on EMMC.
what is the most simple way for a non Linux expert to add the ability to boot from SD Libreelec.
(if I don’t want to re-install the Ubuntu image)
If its possible to have menu on both OS to boot the other one it will be even better.
That restriction pretty much eliminates most of your options IMHO.
The “out of the box” way to get this is to install balbes150multi-3in1 image as per this thread. But you deny yourself the elegant solution if you wont install a new image
I never found a way to do it fully/properly with a Khadas Ubuntu image in eMMC: it always booted with the kernel/DTB from eMMC even if it picked up the rootfs from the SDcard. There may be someone else here who can prove me wrong though.