Ummmm, I'm not sure why everyone in this thread seems to have problems with this. As long as you have a router to disperse the connection, you should be fine. The router assigns seperate local IP's to each computer. I've never had a problem in the past with using 2 computers in my house/apartment to play online with a friend on an internet server. A friend and I once used to play BF1942 online all the time, with him sitting in the next room, and both of us on my DSL connection. Another friend, member kd2777 here, just played CS:S the other night with his brother and brother-in-law, all on the same DSL connection of his. With all 3 of them at his house, and me at mine, we joined many servers that night playing.
It shouldn't matter what game it is, you just have to have enough licenses for each game you are running to play online. For the one who said he couldn't do this with CS:S with his bro, he probably only had one steam account and was trying to use the same account at the same time. In my friend's case and his two friends, they each had their own steam account that was paid for.