I have a 8gb SD card and 64gb eMMC.  I want to put Armbian on the SD and use eMMC for high disk usage activities.   When using the eMMC it will always boot of that device.   You can alter the boot.ini to get it to boot off the SD once it reads the boot information from the eMMC.   I installed Odroid Mate on BOTH the eMMC (to get the boot partition created) and the SD I can get it to use the installed OS from the SD card. Original Odroid boot.ini from eMMC: setenv bootargs "root=UUID=74c