So Rogue Trader has MAJOR co-op issues. We couldn't even get through a single round of combat without it desyncing.
We think we found a fix, by forcing both computers to 60fps, with VSync off.
However, on my machine that causes a lot of screen tearing.
I have 2 monitors, so I tested the other one and, yay, no screen tearing. It is, however, currently set up as a vertical monitor... I'm actually kind of impressed the game runs like this.











