Defining a property without the instance being in the movie

Hi, I’m a bit on the novice side of AS3 and am struggling away. I have one major issue which I cannot fathom.

I have two swf’s which I am loading into a flip book. They will both on the stage at the same time one on each side of the book. I want one to control the other. The issue is when I try to set up functions in one to control objects in the other I get and error about undefined properties. I assume this is because the objects I am referencing are not in the movie where the function lies. And vice versa, when I set up the functions in the movies when the objects are I get an error as the buttons are not defined as they are in the other swf. Can anyone advise me on how to approach this please?

Cheers