TheBrain 8 is not an extension of TheBrain 7. Yes, you can have both installed and yes they both do use the same license information (more on this below), but they are meant to operate individually and are not an extension of one another. TheBrain 8 adds lots of great new features and is quite different than TheBrain 7.
I'm guessing that some of the confusion is coming from the fact that we changed our licensing process with the release of TheBrain 7. Prior to TheBran 7, there were serial numbers for every new version of TheBrain (known as PersonalBrain at the time) and our focus wasn't so much on Cloud Services integration. TheBrain 7 changed all that and Cloud Services integration is a big part of the software and our licensing system is now reliant on this system as well. That is, you need a Brain (online) account setup and need to login to check your license validity. To answer your main question: I am still treating TheBrain 8 as Beta software. I still have TheBrain 7 installed and will use it instead of TheBrain 8 if I am working on something important. Once TheBrain 8 is officially released I will switch to TheBrain 8 as my main.