If you mean a multi-arch iso, It can be easily done using multi_arch_iso.cmd from UUP>ISO and ESD>ISO. Just feed it the x86 and x64 iso, it will offer you a selection of options.
I want to have in one USB the W10 AiO and a AiO of Ubuntu that I made earlier butit doesn't recognize the W10 ISO
Homebrew always fixes all WTK only exports all x64 indexes to the x86 install.wim and puts all on the x86 iso, that you could have done manually. But won't boot x64 uefi.
once i did dual-boot with bootwimx86 and bootwimx64 but i dont know if this support uefi/legacy etc...
They just didn't update the tool to be able to handle the MCT style multi-arch iso's. But the separate x86 and x64 iso would be enough imho, for this purpose.