The latest gaming festival, PAX, wrapped up on Sunday but there were plenty of announcements and demos shown off for the first time.  While Duke Nukem Forever seemed to steal the show, Valve was there too, showing off the cooperative side of Portal 2.  Each of the two characters will control their own set of portals, helping each other through the test chambers.  The multiplayer aspect of Portal 2 is apparently twice as long as the original Portal, ensuring a new level of depth that'll take longer to master.  Check out the 8+ minute walkthrough of four multiplayer levels to amp up your anticipation for its February 2011 release date.