This is a triumph.
Valve showed more Portal 2 co-op gameplay at the 2010 Penny Arcade Expo last weekend. They say the co-op mode will be double the length of Portal 1 and link players via internet, local connection, or on the same TV/monitor using split-screen.
Four excerpts are below, demonstrating the ping tool (shows your partner where to aim a portal instead of saying “over there, no, there”), the return of the infinite jump, the new “hard light bridge”, and some of the gestures, like pressing a button to make the players hug. All games need a “hug” button. Right next to the “rip out somebody’s intestines and fashion violin strings for the tiny violin you’ll play for them” button.
Here’s the ping tool in action:
Hard light bridge: