Preparation:
Load up VS Survival with a friend.
Set every setting to the lowest possible (difficulty and waves).
Choose your favorite class.
Start the game.
Each player gets to play a round as the Zeds.
Player A whom is not a Zed does nothing and lets himself be killed.
Next round the roles are inverted.
Player B now remains afk and is killed.
You can also play two games. One time Player A wins every round and Player B stays afk the whole game and afterwards the other does. This way you also unlock: