Instead of requiring manual deactivation, there should be a similar "log out of all other computers" feature that Adobe Creative Cloud's license managing system uses.
Sometimes, you're not aware that there was an installation on Parallels, for example, since that might have been a one time use, testing out one feature that turned out to not work, and you end up having to setup bootcamp, etc.
With the current system, it would be helpful if the user knew which (operating system) computer names are currently being counted for on the 2 activations.
Use case where it is absolutely impossible to manage license on user end: indie developer's hard disk drive fails - both Mac and Windows images on the same HDD (counted as 2 separate Unity license installs) are no longer available. There is no way for the developer to deactivate on their end.