20ish years ago I was on a team that was working on the PC version of a popular sports game. For the multiplayer mode (2 players only), the way the game worked

20ish years ago I was on a team that was working on the PC version of a popular sports game. For the multiplayer mode (2 players only), the way the game worked both instances had to maintain the exact same internal state. For debugging, one of the things we used to do was connect two machines together via the parallel port and let them play each other for hours. It was slow, but still far faster than ethernet because the latency was so much lower.