Book with two characters, Will and Liam

[zoefruitcake]

I'm a librarian so it is my job to try and work out what book someone is trying to describe from useful things like the name of a secondary character or the colour of the front cover. I usually do pretty well but I am stumped trying to remember the title of a book I read once. It is an English book about two men who went to school together, one called Will and the other Liam. The chapters alternate and are headed with the name WILLIAM with either the Will or Liam part in bold. One of them moves next door to the other and finds a hole in the wall and starts watching the other from within the walls. Does this ring any bells with anyone?

Cross posted