I'm sorry they are continuing to do this. They are getting odler now, and maybe trying to establish dominance with the boys? With each other? How old are they now? (I forget)

Have you recently changed litter? Maybe they don't like the new brand. Cats don't like change AT ALL, and moving the boxes, while changing litter brands could really upset them. Maybe just moving the boxes was enough change to upset them.

How many boxes do you have all together? The girls are gettign bigger, and now may feel they need more room and more choices. I fluctuate boxes based on how many cats I have. Right now, I have 6 boxes for the 7 cats. I know I shold have more, but I scoop 2-3 times a day. When I had 4 cats, I had 3-4 boxes (more boxes for when they were having bouts of "tummy issues")