Ibb and Obb is a PSN (for PS3) platformer built around the idea of who characters who can occupy either side of the screen, divided by a line. In essence, they can walk above or below the ground, turning upside down when they pass through certain portals.
Players have to cooperate, sometimes on opposite sides of the screen, sometimes on the same side, in order to pass obstacles -- and when one dies, the other does too.
In the sequence captured above, players each have to run past the three enemies, then jump over the sawblade guy. The first player to get across has to wait, avoiding another enemy, for the second player to arrive, at which point they have to cooperate to boost one up through the portal. Then the enemies can be killed from below.
I watched a pair of GDC attendees work on this sequence for a good five minutes. When they finally got through it, they got up and left, leaving me alone to check out the game for myself. Which would have been great, except I was alone, without a co-op partner. This brief video probably captures the experience better than my own playthrough and writeup would have, anyway.