We start in parallel Perspex tubes, you and I. I wave to you. I don't see if you wave back, because I'm too busy admiring my own stiff, unconvincing wave animation in third person. I can tell you you're fat, though, and trembling slightly.
We're released into two corridors, separated by glass. We both get our portal guns, but you hit a dead end on your side – even with a small window between us, there's only one place you can put a portal. No use – we can both make an entrance and an exit, but mine doesn't link to yours.
Luckily, you have a suggestion: you place a marker on your wall. You're telling me I should put mine there. It's not a bad plan – I can shoot it through the window, then create an exit on the other side of your dead end. But do I want to help you? I did kind of like what you wrote in my birthday card this year. Bwoops! Bwoops! I let you out.
As soon as I help you past your dead end, I hit one on my side. Um, help? The solution is the same, but vice versa – I place a marker where I need your portal. Come on. Forget what I wrote in your birthday card – help me out. Bwoops! Bwoops! Thanks!
We're together at last. We trundle to the exit, and GLaDOS judges us. “Keep in mind that – like Albert Einstein and his cousin Terry – history will only remember one of you.” I look at you, you look at me. I'm not Terry.
In the next chamber we have a Thermal Discouragement Beam to play with: a searing red deathlaser. There's a cuboid prism on the floor that can redirect it and – oh, charming. You've set fire to me.
The Discouragement Beam can't actually kill us, but thanks for trying. When you're done, we have to shine it through two thick walls to hit a charger on the other side – it'll power up the door. This sounds like a job for: two guys playing Portal 2 co-op! Bwoops! Bwoops! I put mine either side of the first wall, so the laser shines through it. Now you – bwoops! Er, not on that wall. Bwoops! Bwoops! Eureka. The door opens.
Every puzzle in Portal 2's six-hour co-op campaign requires two people. There are no clever shortcuts for one of you to do all the work. If there were, one of you would do just that, leaving your partner behind like a jerk. Valve learned that in early playtests, so they made it a requirement that no puzzle can be solved alone.
What I'm trying to say is, come back. You can't do that bit yet. I have to put a portal here – bwoops! – to extend that light bridge. The bridge is pure energy projected from a generator, so it goes right through portals until it hits a wall. With four portals between us, we can redirect it all over the place. Use yours – bwoops! – to extend it around that corner – bwoops!
Great. Now just as long as you don't – bwoops! Splash. God dammit.