I've got Rawhide, F31, F30, a couple of F29 installs, Manjaro and Ubuntu 19.10 on my
external HD (UEFI system). The "trick" is to mount the same efi partition at
/boot/efi for all of the installations. All Fedoras share the same /boot/efi/EFI/fedora
directory, whereas Manjaro and Ubuntu each get their own directory under /boot/efi/EFI.