Licensing for dual booting different versions of Mac with Parallels

I am needing to purchase a Parallels for a Mac, however I have a question regarding the licensing.
Parallels will be used on 1 laptop, however this laptop needs to dual boot and have 2 different versions of Mac OS installed, each needs Parallels installed. Of course only 1 instance will be used at a time, but in a given day we might need to swap between the Mac OS boot drives. Would I require 2 parallels licenses for this or would 1 license work? I'm not sure if the license is tied to the hardware or the OS.