On Fri, 14 Sep 2018 10:19:03 -0500
tclug-list-request at mn-linux.org opined:

> Message: 1
> Date: Fri, 14 Sep 2018 06:54:02 -0500
> From: o1bigtenor <o1bigtenor at gmail.com>
> To: TCLUG Mailing List <tclug-list at mn-linux.org>
> Subject: [tclug-list] Boot setup question
> Message-ID:
> 	<CAPpdf5-C3_TAJ+qnRgqO3GR37hVbh2iR84RMbxwVOSPaofy6Gw at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
> Greetings
> I have had it working before but I can't seem to get things right this time.
> I want to have two different copies of debian on one box with the
> choice of which when I boot in.
> Wanting to have available both debian stable (9.5 at present) and
> debian testing (10). This is so I can experiment with software - - -
> adding it and if huge errors result or are caused - - - no real biggie
> because its not a main working box (like the server or my main
> computer). Doing this because I really have gotten to hate having a
> main box down for even a few days because software that I loaded and
> installed caused me to bork the system (I've done this more than a few
> times!!! grin - - -but 'learning' isn't always a barrel of fun!!)
> So I've installed both of these systems (more than once each) they
> have their own partitions for everything but boot and efi yet I'm only
> seeing one system available on grub (depending upon the last install
> as to which). So I'm doing something wrong!! I tried using grub
> updating tools (# os-prober) still no joy. The web pages that I'm
> finding seem to be for an older version of grub and, as usual, I'm
> finding man pages are like reading cuneiform (which I find
> unintelligible).
> This is likely something quite simple but I'm just not seeing it - - -
> please - - - some ideas/pointers?
> Dee
> ------------------------------
Going off of memory here and it ain't what it used to be but try this,
When installing first OS, install grub to disk with /boot mounted.
Then install second OS with grub installed to root partition with /boot
not mounted.
Then copy images from second OS's /boot folder to /boot partition and
copy paste menu stanza from /etc/grub/grub30 to etc/grub/grub30 in
first OS.
Don't forget to run grub-mkconfig -o /boot/grub/grub.cfg
to update the boot menu after adding menu stanza above.

Or thinking about it reverse the order of installs above and grub will
probably find the other OS on its own.