Well, I simply tried the brute force way: I let the tool create a dual-architecture ISO and examined it for a bit. It contains a regular Windows bootloader with entries for both x86 and x64 versions of the setup, residing in folders instead of the drive root.
Then I simply removed the original x86 and x64 folders, because they contained only a single edition (going by size, anyway). In their place, I added the whole contents of the regular MSDN ISOs, which contain both editions.
I then created an ISO image using the
oscdimg tool (available here):
oscdimg -m -o -u2 –udfver102 -bootdata:2#p0,e,bG:\WinISO\boot\etfsboot.com#pEF,e,bG:\WinISO\efi\microsoft\boot\efisys.bin G:\WinISO G:\Win10.iso
(Command line courtesy of this blog)
Important: The source (and possibly destination) directory must not reside on a volume that is ReFS-formatted and/or hosted on Storage Spaces. Otherwise,
oscdimg will fail with
Error 87: The parameter is incorrect.
Both architectures installed successfully in both BIOS mode, the x64 version worked with UEFI, too. The x86 version is, however, not bootable in UEFI mode. This can probably be fixed somehow. The original x86 ISO is UEFI-bootable. (Edit-addition/correction: Various sources suggest that under UEFI, windows/setup kernel must match the underlying processor architecture, ie. x32/86 Windows 10 will only install on an x32 machine, and x64 only an an x64 supporting CPU. Seeing as you don't often see UEFI x32 machines that doesn't seem to be much of an issue. See here)
Because I don’t think Microsoft will mind, I zipped the base layout of the disk, it’s available here (19.4 MiB). That means you won’t have to download a dual-architecture ISO to get started. The archive doesn’t include
oscdimg, so you’ll have to get that from the page linked above.
(Addition: This Multi-boot (x64/32, Pro/Home) ISO method also works for a bootable USB drive. You can make such a drive using the various popular methods, and then just replace the files/layout with the same one used for this ISO. Some care must be taken though if you want a USB drive that boots both traditional MBR/Bios and UEFI, as the UEFI requirements can be a bit picky.)