I bought a cheap “broken” gtking box wich I repaired, this box had a 512gb EmuELEC card wich was not working and a 8gb CoreELEC card
Well with the 8gb card inserted the box always boots into Android, and then you have an apk wich reboots the box to the microsd
Well, I downloaded the last CoreELEC 21.0 image and flashed it to the 8gb card (It had CoreELEC 19 originaly instaled). After that the box work as any other box, always boots from microsd if it is inserted.
So that been said I think you can make some configurations in the microsd to force android to boot always first and ignore the SO in to the sd
It is an interesting option, at least for me, so you can always boot to android and then go to CoreELEC as if it was the kodi apk but with the power of CoreELEC