To practise "versus" as a single player special infected.

R click on the L4D2 game start icon.
under the "General" heading click "Properties" and
"Set Launch Options"
add, without quotation marks
"+allow_all_bot_survivor_team 1" click ok

In main menu of L4D2 click "Options"
then "Keyboard/Mouse" and ENABLE "Allow developer console"

In Main menu of L4D2 prior to entering gametype ie versus, or campaign etc

press the button just beneith the Esc key, i'll call it the "Tild" key
and enter the following commands.

" sv_cheats 1 " and submit
then enter " sb_all_bot_game 1 " and submit
then " sb_unstick_0 " submit
then " vs_max_team_switches 99 " and submit
then " director_force_versus_start " and submit
then " director_no_survivor_bots_0 " and submit
finally enter the map code ie. "map c5m1_waterfront versus" and submit.
game should start
Use the "M" key, by default, to select to spawn as either special infected or human.
I have found spawning as human first and moving to get the bots out of the saferoom works before then spawning as a special infected.
To choose the type of special infected to spawn as - while in the game and as an unspawned special infected, move to a safe place away from humans and do the following.
Hit the "Tild" key and enter the code of the special infected
- ie. to spawn as a hunter use "z_spawn hunter" submit and re enter game.