It may also be of interest what your grub menu entries look like. The way you describe it, @johnrcox, there isn’t even a menu entry for openSUSE, so something must be going wrong when creating (update-grub) it. As I’ve learnt recently, you need to be careful with update-grub, when TPM-backed full disk encryption (TPM/FDE) is enabled, so DO NOT RUN THIS, IF YOU HAVE TPM/FDE enabled 
sudo update-grub
Usually the output of that command should already tell you, if the menu entry for openSUSE is being created. Otherwise, a look inside /boot/grub/grub.cfg should provide further clues.
(edited to put code where code belongs 
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Please put all log and other console/terminal output in pre-formatted code blocks (open a new line and click the </> icon in the editor first, then paste into the new element), ideally wrapped in a “Hide Details” element, if it’s more than just a few lines. See items 4 and 5 in the Posting Guide.
Optional helper script
If you are so inclined, I took it upon myself to provide a little helper script for that. 