Yeah you'll need two copies of each to play at the same time on the same server.
And you can choose to play on the same team and try to coordinate things. As I said, I played a lot of DoD:S with my bro so we would stick together and cover each other, etc... Pretty fun!
If you're only interested in DoD:S, then yes that is all you need. It is $20 through Steam, though I also seen it retail packaged with CS:S for $40. I think it is one of the best $20 you'll spend on a game.
Also, we had to update our firmware on a Netgear MR814v2 router since it would not less us join the same Source server. Like if my bro was playing and I tried to join, it wouldn't connect to the server. A firmware upgrade fixed this, though you still might get a message about 'ghosting an IP with another player' but this doesn't seem to affect anything. A Linksys router had no troubles with both of us connecting to the same server, but that firmware was already new anyway.