The Chronicles of the Imaginarium Geographica, is a series fantasy novels involving John (J.R.R. Tolkien), Charles (Charles Williams), and Jack (C.S. Lewis).
What are the limitations of a work like this?
Could for instance John, Charles, and Jack go to Narnia?
Could you take an outside character (one of your own making) and have him/her somehow find their way into Middle-earth?