How to play left 4 dead offline versus

If on xbox 360 go on versus mode play with friends and create your own lobby get a second player choose your campaign and u should be able 2 play.

Here is another way (for PC):
First go to Keyboard/mouse and then enable developers console.
Then open a single player game
type in these codes:
sv_cheats 1
sb_all_bot_team 1
sb_unstick 0
jointeam 3
Then you should be able to play as infected