refind will scan all your partitions for EFI bootable things; if you have two ext4 partitions each with a Debian on them, and each has a Linux kernel in /boot,

refind will scan all your partitions for EFI bootable things; if you have two ext4 partitions each with a Debian on them, and each has a Linux kernel in /boot, it'll locate them both and you can boot either. Which sounds like what you want.