Hi - I think it would be nice, if a dual-boot computer only counted as 1 licence

Well technically one license = one operating system monitored. It would be very difficult to handle like this, also the operating systems have unique identifiers that are used to tie your computer to your account so that no conflicts will occour. I don't think it's a good idea. If you have many dual boot computers why not consider purchasing a subscription, it's cheap and it converts your existing 3 computer licenses into premium so that you can enjoy all the subscriber only features like server modules, screen view, webcam view.

Also have you tried some of the avaiable plugins ? Check them out here.