I have read on the website that Avast licenses are per computer.
Does that really mean that if I have 2 Windows operating system on a computer, 1 license in enough for both of them?
I think you need two …
You may contact support and ask, maybe they have a good deal 
> https://support.avast.com/support/tickets/new
As it is now, you need 2 licenses.
One for each installed Windows version.
You can also use the Free version on as many computers as youlike. 