My assumption is that CoD 5 will allow you to play over the lan with just one game. All of the others have allowed you to do this, but you will not be able to play online. Most likely you will not need the CD to play the MP portion of the game (if treyarch follows the same patch as infinity ward), but I am unsure where coop will fall in this area since it is a first for the series (it may require the CD like the SP usually does).
Left 4 Dead on the other hand will most likely have to be activated through steam, so you will need 2 accounts and 2 copies of the game to play it. This is just what I assume. (as proven below, this may not be the case and you may be able to do it.)
If you are going to be doing this alot and maybe even want to include another 2 people or play online with many others it probably would be best to buy 2 copies of the games for the LAN. I usually do this for games that I play on both my laptop and desktop, so that when a friend comes over they are able to play with me.
Best,
3Ball