Like I said, it is somewhat non-trivial, but you can add additional menu entries to GRUB that will target other operating systems on other partitions or disks.
There is another user who is attempting that with another GRUB bootloader (in his case, one that was installed by CatchyOS) here:
He said he accomplished it by adding a 40_custom file with a custom menu entry definition.
I also use a lone bootloader to load multiple opertating systems, but I do it with systemd boot, which is a completely different boot loader that works in a completely different way and it only works on UEFI systems.
I know it can be done, but I haven’t done anything like that with GRUB for ages and about all I can do is point you to the documentation.
If you want, you could try collaborating with that other user who is working on this same problem.
Good luck. 