I’m going to upgrade from Cubase Pro 8 and Absolute 2 to Pro 12 and Absolute 6 should I delete Pro 8 and Absolute before the new download?
Cubase 8 and Cubase 12 are independent applications. The Instruments in Absolute will behave the same (for example HALion 6 vs. HALion 7) or they will update.
So you can delete the old application before or after installation to save some space. On the other hand, the applications are not really large. The libraries take up space and they are not doubled. So if you want to keep the old one because of older projects, you can do so.
I would suggest leaving the older version of cubase installed, there are a few VST instruments that have been removed from 12 that you may need to be able to play older projects correctly. I.e Mystic, Prologue, Specter, LoopMash