Write it out once, (more) save to doc, copy n paste my friend.
A "Twinner" is a person in another world within the multiverse that is a twin, both in looks and spirit. They are the "you" of that world so to speak. Places have Twinners too, and Derry is the Twinner of our Bangor. It is important to note that in the world where the Losers live, Bangor is just down the road from Derry.... /but/ that Bangor is very different one must presume. That is because Derry is a reflection of our Bangor.
It isn't unlike the fact that Eddie Dean comes from a New York that isn't our New York. Certain landmarks of note are in the wrong place from our perspective... or his looking at ours.
The New Yorks are Twinners of each other, but a Twinner is not an "identical" twin. To some degree it works like this, the multiverse is vast place and in some respects it isn't unlike a House of Mirrors. Reality reflects reality reflects reality.... but the reflections aren't perfect because the mirrors all have different imperfections. Think of funhouse mirrors. Moreover, it gets harder to see the detail when you look at a reflection in a reflection in a reflection. It isn't a perfect metaphor but you get the idea.The Multiverse (or the many levels of the Tower) reflect each other and in some hard to understand interact. Even the keystone world is affected by the reflections of itself. It is important to note that not every world has a Twinner of every thing or every person. I won't go into that level of philosophy to answer the original question.
