if we download windows 10 using "Media Creation Tool" their's option to download both architect x86 and x64 on a single ISO, it's possible to make same with windows 8.1 or windows 7 OR All windows edition on single ISO. yes i knew its possible with Dism and make a large wim or esd but windows 10 work differently. when you open AIO ISO of windows 10 there are two folder that hold two architecture.
I played around with this not long ago, posted about it somewhere. The BCD can boot just boot anything from anywhere but the Windows Setup files seem hardcoded to only run \sources\install.wim. Windows 10 Setup files seem hardcoded to \x86\sources\install.wim and x64\sources\install.wim as well as \sources\install.wim. Change any of these and you get errors in setup (Media driver missing, no index found etc.) I tried changing the BCD path to several different places such as W7\x86\sources\install.wim. x86\W7\sources\install.wim etc and they all failed. Just off top of my head, what you could do is edit the BCD and rename the wims: windows7x86.wim, use the MCT tool for the file structure etc and try and get the W10 setup files running them. I gave up on this way and went for a direct WinPE type installation in the end.