Identify iButton location
You'll need to install a 1-wire GPIO chip at each reader, hooked up so that you get a contact closure when an iButton is physically inserted. Then, in addition to reading the iButton itself, you query the GPIO chips to determine which reader actually has an iButton present.

There may still be some confusion if two or more people insert their iButtons into different readers simultaneously &mdash; you won't know which one is which. But the window of opportunity for this kind of problem is small, and if it does occur, you can probably simply not unlock any doors, and let the users retry their buttons. It's unlikely that they would have the same timing two or three times in a row.