That is a problem I have with games that require Steam. Yes you can install the game on two different computers. However you cant be logged in to the same steam account on two computers at the same time. This means that if one person is playing a different steam game on one computer or even logged into steam for another reason, you cannot play MW2 on a different computer. I know there is an offline mode, but I dont think it will work with all games, and if the saves are in the steam cloud, I think the saved games could get messed up too.
Another thing that really made me angry about MW2 was that you could not have multiple characters with the same game. For instance in dragon age, one person could play character A and a second could play character B. The two players could play at different times and not interfere with the other player's saved games. I could never find a way to do this in MW2. You could only have one character at a time, which basically meant that two different people could not play even at different times.