Unless the hardware for both is identical, licensing will not be your biggest issue. Windows doesn't like to boot from an external disk at all [Win2Go was abandoned]. Each time you try to boot to dissimilar hardware, it will either fail, or have to reinstall a different set of drivers.
A retail Windows license can be used on more than one computer, though not simultaneously. Technically, you will meet this requirement. Practically, however, you will need to reset your license each & every time you move the boot drive, or just run it unlicensed [this is what I do with Windows VMs which are temporary & I don't care about the slight reduction in customisation options.]
Your call, but I'd consider it more trouble than it's worth.