Tonight my older son and I worked through a level and a bit of Portal 2 co-op. It's interesting to see how we work together - we do most of it without any explicit communication. Typed messages back and forth are more likely "oops" than anything else. (Especially when one of us drops the other into the sludge. "Accidentally.")
He's scaring me now - he's figuring out the puzzles faster than I am. Good thing he's on my team.