You will need three players for it to work. Go to a versus game and have two people on the infected team.

The last person will need to be on random. Then you press start and the last person should be on the infected team.

The survivors are now bots and will attempt to get to the safe house. If you kill them or if they get to the safe house, the game will not end.

You can wonder and fight to the death. Since the game wont end you can practice glitches,cheats, and secret hiding places.
This cheat should only be used in the first chapter or else the bots will never leave the saferoom. If it doesn't work then try making a different person random.

If still doesn't work then try having one person play splitscreen with another person.