Biped

Biped is built for two people specifically, and you can feel that in the design of every puzzle. Your robot's legs are controlled independently, which means coordinating movement with a second player isn't a feature, it's the game. Some puzzles require you to physically anchor one player while the other swings across a gap, which sounds straightforward until you're actually trying to do it and both of you keep letting go at the wrong moment. It's funny in the way that physics co-op games always are, but it's also genuinely clever. Shorter total playtime than the games above it on this list, but every hour of it is designed around the fact that there are exactly two of you.