codice:
box.loadMovie("uno.swf")
box.uno.swapDepth(2)

box.onPress=function(){
     this.loadMovie("due.swf")
     this.due._alpha=0
     this.due.swapDepth(this.uno)
     this.uno.onEnterFrame=function()
          if(this._alpha>0){this._alpha-=2;_parent.due._alpha+=2}
          else{delete this.onEntereFrame;}
     }
}

L'ho scritto senza provarlo provalo tu