A single 1tb for OS and a separate 2tb for games is always a good idea. Windows borks and needs reinstalled, you don't lose your game installs.
I haven't had a single drive only in a PC for um... over 20 years now. For that reason. Windows doesn't fail as much anymore as it used to. Its easily over 10 years since I had to reinstall windows because of something it did.
And the game installs bit depends who you buy off, Microsoft games still need to be reinstalled, Steam not so much. I don't know about the game you play, it used to be easy to just redirect the launcher. I haven't looked at Bnet for that info.