Easy as pie!
Just take two shots...one with helper's hands holding the head steady then one holding the arms underneath. Paste one on top of the other (I chose the one with the best facial expression for the bottom layer, then just pasted the other image on top). Then all you have to do is erase back the top layer to reveal what you want to show through from the bottom!
A safer way to do this so you don't have to worry about messing up is with a layer mask and just paint back what you want to show through with black. I just erase :P
You need to do all this combining BEFORE you edit your image. Then just flatten and process as you normally do!
Hope this helps someone :)