Dual boot DietPi with LibreELEC


is there any way to make DietPi work together with LibreELEC through dual boot.

My goal is to have DietPi on eMMC and LibreELEC on sdcard on my Rock64.
I tried to follow https://github.com/ayufan-rock64/linux-build/blob/master/recipes/dual-booting.md but for now without success.
I edited the extlinux.conf accordingly but the boot menu is not showing on boot. The default boots after the given timeout, so the changes in extlinux.conf are not ignored. But it looks like there is no signal on the monitor and I can’t make a selection.

Maybe anyone have a suggestion or a solution.