Some spoilers -
Somewhere in the beginning, near a train, one player goes into a monitor room with a screen that shows various cameras you can switch between, and a set of buttons.
The setup is straightforward, one person stays in that room and presses buttons to help the other person progress. Sadly, when I was in the monitor room the camera renderer clearly broke - it froze on a still image of the other player leaving the first puzzle that required buttons to be pressed, and I could press the button to switch views, but the camera kept showing the same still image.
We weren't sure what to do for a while, restarted, then my girlfriend was in the monitor room and everything worked fine for her - but when I tried being in that room it broke again.
We managed to play through it by looking at her screen to see if the buttons would do anything, but it was very unclear.
The parts that weren't broken were nice though.