If one of your children has an injury that requires something special -- like an ace bandage -- then suddenly that same part of the body hurts on your other child's body too. I had to send both kids off to school today with ace bandages on their feet. Left foot on both of them.
Unfortunately, this phenomenon doesn't extend to more useful sympathies -- like when one child feels like being cooperative and helpful, the other one does too. Or when one child decides she is sleepy and wants to quietly nap, the other does too. Never.