That's just it. There is no simple answer for this. You'd need to do some significant surgery on the two files so that the same table in one of the two files serves as a "contacts" table for both files. That's not a trivial task to take on--especially if the person asking is also still learning how to work with FileMaker databases.
There is a Business Productivity Pack that is offered. I think those files were designed to share tables from the start so you might find that they may offer what you are looking for without having to redesign a pair of starter solutions to do so.
If you still want to take a stab at linking these two files, let me know and I can at least outline the steps for you, but you may have already read such a description since you indicate that "this has been asked a million times..."