I once asked a similar question long back (can't seem to find that old thread now), but apart from the above query, my issue was more like related to the LOGIN credentials on both these sites/forums.
Tom's Hardware forum's current login-in username/password was not working on 'Tom's Guide Forums', website, so I had to create a NEW one using the same EMAIL address I signed in here.
Both sites used to be linked but now are separate, however the moderation teams and management are the same.
As for the difference, Tom's Hardware is pretty specifically about PC hardware, Tom's Guide is a tech site for stuff like mobile devices, consoles, etc.