As Ray says, each 600EX can act as a transmitter or receiver, but if you always want the hot shoe side to stay dark you can save a bit of cash/weight/bulk and get a YN-E3-RT or ST-E3-RT to stick on the camera, which is just the transmitter/receiver part without the flash head.
They take 2 batteries instead of the 4 needed for each flash. I think the YN version has an AF-assist lamp that sort of works, ish... And if I remember rightly, the ST (Canon) version doesn't have the lamp.