Multiboot
Articles about multiboot on EFI:
To enable EFI booting, we need a GPT disk, a /dev/sda1 that is VFAT and a system that supports EFI booting (not MSDOS/MBR)
Usually when I first get a brand new computer, I use a LiveCD (more like a LiveThumbdrive, e.g., Ubuntu LiveCD) on a thumbdrive to boot, open a Terminal, install gparted and run gparted to partition the new disk into as many partitions as needed (about 60 GB each).
To install a new Linux distribution, on any working Linux OS download an .iso (e.g., "wget http://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/weekly-builds/amd64/iso-cd/firmware-testing-amd64-netinst.iso") and use the dd command to transfer the *.iso to a new installation thumbdrive via "dd bs=4M if=firmware-testing-amd64-netinst.iso of=/dev/sdX conv=fdatasync" (change X accordingly). Boot up the new computer from the OS in the new thumbdrive, and choose a partition /dev/sdaY (change Y accordingly) on the new computer to store the OS. The whole installation should takes ~20 min.
To install two versions of Debian, say Debian12 and Debian11 on the same hard drive say /dev/sda7 and /dev/sda8. If we are able to boot up one of them (since EFI boots from the latest /dev/sda1/boot/efi/EFI/debian), then simply run "update-grub" to detect all installed OSes on the system and produce a new boot menu grub.cfg. Upon reboot use this new boot menu to boot any pre-installed Linux OS.