FWIW, I’ve seen people report success with getting one Grub loader to load another’s configuration by adding a small /boot/grub2/custom.cfg file to the one that normally boots.
Disclaimer, I do not know how likely that is to work in all cases. You might have to load additional modules (e.g. filesystem drivers) in some cases depending on what is available and what is needed.
Edit: Note that you do not need insmod chain when using the configfile .../grub.cfg method of loading the second OS configuration. The user in that other thread left that extra line from an earlier example, but it wasn’t needed.