Is there a way that Grub can be set up to boot two different versions of Windows in addition to 15.6?
The easiest way is to give both Windows installations their own EFI partition.
It may also be possible to edit /etc/grub.d/40_custom by adding chainloder followed by the Windows UUID and then /Windows/System32/winload.efi.
Windows NT+ always had its own built-in boot manager. Grub chainloads the Windows bootloader, and Windows then produces its own OS chooser. Whether UEFI has changed this in any significant way I’m interested to know. I suspect not.
Good question. Looking at random example of Windows EFI boot entry
Boot0002 Windows Boot Manager HD(1,800,32000,4462fa04-dc5d-4536-8f23-8c34bc638faf)File(\EFI\Microsoft\Boot\bootmgfw.efi)WINDOWS.........x...B.C.D.O.B.J.E.C.T.=.{.9.d.e.a.8.6.2.c.-.5.c.d.d.-.4.e.7.0.-.a.c.c.1.-.f.3.2.b.3.4.4.d.4.7.9.5.}...a................
The content after file path is parameters that are passed to the program. Now, B.C.D.O.B.J.E.C.T.=.{.9.d.e.a.8.6.2.c.-.5.c.d.d.-.4.e.7.0.-.a.c.c.1.-.f.3.2.b.3.4.4.d.4.7.9.5.} looks like it refers to the boot entry in the Windows BCD store. So, it may be possible to have two GRUB menu items that chainload bootmgfw.efi passing different BCDOBJECT parameter. I have not found any reference or other description of parameters boomgfw.efi takes after cursory search. You will need to research and experiment yourself.
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