Thank you for those hints, it's good to read that it is working. Many details to consider though. I ended up without a ROM with kexec-hardboot. Instead I booted into recovery and copied with adb pull /sdcard/TWRP TWRP the whole backup to the pc hard disk. Flashed something else, created a backup and completed the same process. To boot into the first OS again, copied the backup back adb push TWRP /sdcard/TWRP and made a restore (same process if I want to boot the second OS).
Disadvantage is of course, that it takes a bit time to switch roms (but it takes not too long actually). However I can use whatever combination I want (and there will be more space for one rom).