I'd try to get more people in on killing floor, myself. One of the great things about it is that it's so well optimized that it'll run at a smooth and high FPS on basically anything, and the fact that it goes on sale frequently for like, $2.00, makes it the perfect game to gift to friends who might not otherwise play multiplayer computer games. With two people it's still ok, but it's really at its best with all 6 (I haven't played in a while, I think it's 6) and something like the fake players mod running (adds fake players past the cap to increase the spawnrate).
TF2 is another fun one with friends, but I've only played it with other people who are already good at it, so I dunno how much fun trying to introduce it to someone would be. Late night valve servers are hilarious with friends, where astoundingly derpy things are bound to happen with some of the more 'special' players. You can also join random servers running weird mods, which is also fun.
Terraria has multiplayer, but I haven't played it, so I can't particularly endorse it. It's a fun game though, and goes on sale often (may be a while, since it just updated).
There are of course several billion MOBAs out now to choose from, but I'd avoid letting them eat your soul if they haven't already.
Starcraft II has some fun mods, but it's expensive, clunky, blizzardly, and otherwise useless if you don't like RTS.
Dungeon defenders is ok, but you've already mentioned orcs must die, I'd say that's the better game.