Samuel Sieb composed on 2024-06-04 15:03 (UTC-0700):
You don't have an EFI partition on this disk. It's not even a GPT partition table. So both installations will be fighting over the EFI partition on sda. And there's no easy way to add a boot entry for this installation.
A multiboot GNU/Linux PC needs only one bootloader. Maybe OP's configuration can be improved through some additional information: https://forums.opensuse.org/t/how-to-have-a-custom-uefi-grub-menu-for-a-multiboot-system/133541