When is left 4 dead and left 4 dead 2 characters going to meet

(CAUTION: This answer contains information that may spoil the campaign(s) if you have not yet played them.)

They meet in between the Left 4 Dead 2 campaigns Dead Center and Dark Carnival in a download-able campaign called "The Passing", which is soon after the campaign for both Left 4 Dead 1 & 2, The Sacrifice. The Left 4 Dead 2 survivors are driving through an infected town and the bridge is out, Francis or Zoey will tell you that you have to gas up a generator to get across because it has no energy left from when they raised it. Louis has a hurt leg so they can't leave him alone, and Bill got taken out by a few tanks, you will see the bloody result in the room of the scene. In The Sacrifice, where you play as the original survivors, any one of the survivors can sacrifice themselves to restart the malfunctioned generator to raise the bridge, but it will always appear as Bill in The Passing.