I actually solved my own problem! It turns out that "dm.exe" just doesn't really work with DOSbox, I guess. I tried a newer version of a similar app with two different computers and I was able to connect a null-modem game successfully.

Hello, this is my first post on this forum. So please forgive me if I am posting this in the wrong place or if it has already been asked (I really tried to look, honest!) I am trying to recreate this nullmodem experiment with Doom Shareware. https://www.youtube.com/watch?v=nAHGD6JrENI I hope that if …