Copy visible pixels of 2 Sprites


I want to copy the visible pixels of 2 sprites whitin a new Sprite.
The top Sprite is transparent in the middle so i have to see the sprite beneath it.
But the middle is always white and not transparent can somebody help?

function cloneBitmap(myOldClip:Sprite, myNewClip:Sprite):void
 var myBitmapData:BitmapData = new BitmapData(myOldClip.width - 1, myOldClip.height - 1, true);
 var bmp:Bitmap = new Bitmap(myBitmapData);