Have you been using the same memory cards between cameras?
Perhaps the cameras are having a problem with the memory card and thus refuse to shoot. How is your custom setting configured for the camera to operate with no memory card? Usually it will give an error on the LCD for a bad card. I've had that happen and had it go away after turning the camaera off and on. It might be worth a try and it can't really hurt. Put the card(s) into a card reader on a PC/MAC and do a disk check followed by a low level format. Then format in camera.
Just a guess.
I've taken up the habit of occasionally doing the disk check / format routine for prevention.