UESP's article on the Atronach Forge seems to imply that there's only one, in The Midden at the College of Winterhold. However, I seem to recall stumbling across another one during a quest at some point. I'm having a hard time remembering when/where this was, so it's perfectly possible I'm mis-remembering.
Can someone tell me if I'm crazy, or if there is actually another Atronach Forge that's just not been documented on UESP? Could it be not documented because it's not usable?