Using a Macbook Pro as a second monitor for another Macbook Pro
so we'd like to watch the same movie, synchronised, on both computers.


You can leverage the Screen Sharing feature built into macOS to achieve this. You'd just need a network connection (which you happen to have via Wi-Fi) and would NOT need any cables whatsoever.

To use Screen Sharing, on the MacBook on which you'll be playing the movie, go to System Preferences app → Sharing and enable Screen Sharing.



Once configured, the other Macs in the same Wi-Fi network would be able to see the configured MacBook and can request access to your configured MacBook.

However, in this scenario, you'd have to resort to using the original Macs speaker. So the audio playback would only be from one MacBook (which would also be the case in an external display scenario).