It's not officially supported right now, but people have found work arounds. Hunt around the steam forums for more information.
Here is one thread to get you started:
http://forums.steamgames.com/forums/showthread.php?t=1852847
Yes you can... Enable the developer's console in options - on one PC - open the console and type;
sv_lan 1
then type:
connect {other PC's local ip}
on the other computer type;
then type;
map mp_coop_start