lay them on their sides rolled up, fitting together like a Ying and Yang sign.
put a empty frame in between them to make it look like a mirror image./. only that it isnt
Have parents hold them laying on their arms, so that the feet are on parents hands.. have them stand close together to get a shot of the underside of their feet ont he parents hands and blacken out in PS everything around it afterwards (draping something black around them will make it easier later)
put them on their bellies, right next to eachother, knees up underneath their bodies.. fetal position and shoot down on them