After it splits you will need to kill both of them and one of them will drop a golden key card. Once it spawns you will need to kill it and make it split. If you decide to obtain it through the Easter Egg you will need to wait till a Megaton enemy spawns. One doing the Easter Egg steps, second you can obtain it out the box, and third you can do the coffin dance side Easter Egg and have a change to obtain it.