First of all you should define which method you use to load one swf to another. On this depends what will be the reference to the top level.
If you used flash.display.Loader
then from MainTimeline of loaded swf to MainTimeline of loader swf (if you load not inside so object but create Loader
itself by code on Maintimeline - reference will be
MovieClip(parent.parent)
Loaded swf root stored inside Loader
instance at content
property, that why there is two parent
. And data-type convertion should used because parent
return reference as DisplayObject
and thats block you from access it properties.