Unfortunately, you can't go back at all. The only universal area for BG2 and ToB is the Watcher's Keep. Once you have moved into ToB you can't access any more of the old SoA areas.
I thought this was a bit weird, but hey, I'm no games designer...
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Paladin in the court of Lady Lioness
"Our deepest fear is not that we are inadequate.
Our deepest fear is that we are powerful beyond measure."