I have never been able to do that but one solution is, to duplicate the av piece you are working on edit the part you wish wish to put over the other and then move the bit into place. But one thing to remeber, a full av can only be 9 pieces big:) I hope this helps you out 8)
First open (click edit) the prop you want to take a bit out of...
Use the selection tool (the dotted square) place the crosscursor in the top left corner of what you want to cut out and drag it untill right lower corner. Then hit CTRL+C
Then open the prop you want to place it in and and hit CTRL+V
Then just drag it to whereever you want it, you will have to edit the background out of it though...