The idea is to boot into a previous snapshot… I suspect the two installs have overwritten something… If you going to use two installs, way easier to use two storage devices… Consider a fresh single install and use snapshots as designed when problems arise.
Oh and increase the size of the /boot/efi partition.
@bzz_bee not sure what you mean, as long as they create their own /boot/efi/EFI/distroname all is good. I use grub2-bls with Windows 11, use the boot menu for windows, system boots to tumbleweed by default. If using grub2-efi then you can set a distributor name for different openSUSE releases.