They are not tied together at all. Each site has its own forum so you need to register on each, this is because each site has different features tied with forum accounts so it's to simply things for the user and makes each site more flexible for the site devs
Hey Rushster, this is GaussWaffle, and I'm but a regular forumer at the SC2 forums here at IncGamers. I've recently decided that I'd like to also start posting in the Diablo 3 forums, and promptly went to and signed in to the Diablo 3 forums. It said I was signed in, but would not allow me to reply to any posts. I tried registering under the diablo forums, but they said my name was already taken (which was what I was expecting).
So, how exactly do I use my forum name from one part of the network into another? Is there some activation I need? Any help would be greatly appreciated