Start a versus map in split-screen mode:

ss_map l4d_garage02_lots versus



If your first player is a survivors, use this to kill them:

sv_cheats 1; kill [your_nickname]; sv_cheats 0



Use this to instantly start the finale generator with the second player:

sv_cheats 1; warp_all_survivors_to_finale; setpos_player 2 6820 5920 20; sv_cheats 0; cmd2 +use

Do not let the survivors die during waves, help them by teleporting them away from the crowds of zombies using this:

bind "g" "sv_cheats 1; warp_all_survivors_here; sv_cheats 0"
(Binds, teleporting survivors to your crosshair, to the "G" button)



(This step might be a bit tricky to get but if I can do it you certainly can aswell)
When the generator needs to be restarted teleport the survivors next to the generator using "G" and use these commands to spin and stop the second players view and restart the generator:

cmd2 +use
(type this first so the second player activates the generator when rotated correctly)
cmd2 +right
cmd2 -right
cmd2 +left
cmd2 -left
(+ starts rotation, - stops rotation)



After that (or maybe meanwhile if your slow) you will become a tank, let the survivors kill you and the truck door will open.

Teleport the survivors away from the truck with "G" and incap them as a another Tank.

Done.

Wait until the score screen is done and the achievment should pop up.

If it didn't pop up maybe a common infected got the last hit on one of the survivors, just try again.

I can't confirm if the achievment pops up when one survivor has already entered the truck, since when I tried it with a friend and the bots got in it didn't work.