We’re noticing a bit of a theme here. Valve likes co-op. The latest game to jump on the bandwagon? Portal 2 – more specifically, the Perpetual Testing Initiative. While the original game’s had co-op play since its inception, the in-game level editor was restricted to single-player only.

Now, thanks to a handy dandy (free!) update, you’re able to design, create and publish your own co-op test chambers.

…for science!

(Psst! Speaking of Portal 2, Player Attack is giving away a replica Portal Gun, just like Chell used in the game. All you need to do is a little social network following, and tell us exactly what you’d get up to if you had a real Portal Gun!)

The Perpetual Testing Initiative for Portal 2 was launched in May 2012, and has since seen more than 170,000 single-player test chambers created and shared among gamers. Now, it’s time to get your friends in on the action. Valve is even making that part super-easy, by giving out Steam coupons to anyone who owns a copy of Portal 2. That coupon? 75% off another copy of Portal 2, designed to be given to the person you’d most like to co-op with (or, perhaps, someone else if your chosen partner already has the game).

The news follows this week’s launch of Mann vs. Machine for Team Fortress 2, which added co-op gameplay to another award-winning Valve franchise.