The cmdline.txt file on the boot partition of the SD card points to the partition to be used as root file system. You might be able to put the debian rootfs in one partition, the openelec in another partition and create a script under both systems to choose which OS to boot at the next boot. The script would then change the cmdline.txt to point to the right partition.
The script might also be used to change the kernel.img and/or start.elf to the appropriate one for the debian/openelec version you're running.
If you want to have this as boot menu you could have the script run early in the startup process of each OS and have it reboot if the current partition isn't the right one.