Thoughts on portals

So can any of you think of a portal story that lets the people from each side actually live on the other? It seems they are mostly separated, with someone from one side going there for a time, but then coming back. Aislinn straddles the line in Wicked Lovely, and a number of characters make permanent moves in Mary Hoffman's Stravaganza series (great series, BTW). But those aren't easy. And mostly people don't stay. Look at Wildwood Dancing: one of the plot elements was that people couldn't stay in certain environments. I wonder why this is? And I wonder why in real life it's so hard to live on both sides of a portal.