The casings were not stacked. The stacked idea came from one of the posts in the Psychic thread made by Bern.
http://www.websleuths.com/forums/showpost.php?p=2849478&postcount=291
There is mention of the cases found being in a pile and that one of the area TV videos showed them. Did anyone ever turn up this bit of video or any stills from it?
I'm going to indulge in a bit of speculation. If the piled casings were all one caliber and were from the smaller caliber gun, I think this might have happened. In case anyone hasn't caught my other posts on this (and with so much discussion, they're easy to miss), I think the smaller caliber gun was a .22. The autopsy reports describe the bullets as copper coated, not jacketed. This is an important distinction, as .22s are the only ones commonly coated, not jacketed. Anyway, if the smaller gun was a .22, it may well have been a .22 revolver. My idea is that the shooter with that gun emptied it into the 2 girls. Then he swung the cylinder open and dumped the casings at his feet so he could reload. The killers may have wanted to be ready to shoot again in case anyone came up before they could get away. The one with the .40 caliber could have simply inserted another clip, but the other, if he had a revolver, would have to empty it before he could reload. This scenario would explain the casings being in a pile without the shooters having to do anything special for them to be that way.
Please note, this is just speculation. If it turns out the pile of casings were from both guns, then my idea is wrong. Same if the smaller caliber gun was a semi-auto or rifle.