Fireboy and Watergirl 3: Ice Temple

Fireboy and Watergirl 3: Ice Temple builds every puzzle around a restriction that never changes: fire kills Watergirl, water kills Fireboy, and both need to reach their matching exit door, so a solution that gets one character through safely can be an instant death trap for the other.

Levers, moving platforms, and elemental pools are placed to force cooperation rather than letting one character solve a puzzle alone, moving Fireboy across a puddle he can't survive standing in usually means Watergirl has to hold a switch open just long enough for him to pass. Playing solo means constantly switching control between both characters rather than favoring one, since a level rarely progresses with only one of them making it to their door. Duo Vikings runs a similar two-character puzzle format built around castle-raiding instead of elemental temples.