On a Running 64 bit programs on a 32 bit system answer there's this screenshot:
What does the installer do if you select Both as architecture?
I don' think there's any way of having a both x86 and x64 system – without it being an x64 system itself. The alternative could be installing two OSes – but I don't see how can a single installer do both and still have a decent UX.
Best Answer
AFH is correct. This screen is for the Microsoft Windows 10 Media Creation Tool. It will either prepare an ISO or a USB device with the installation files capable of both the 32bit and 64bit architecture. Of course if you choose both this will create a larger ISO file or take up more space on your USB device.
https://www.microsoft.com/en-us/software-download/windows10