I think all Steam does for TD1 is launch the UBI game launcher. I don't think the game is truly run through Steam.
I have two copies of the game. I used to different UBI accounts instead of Steam. I often run them simultaneously on two computers sitting side by side. I have gotten fairly good and playing them both. I have done legendaries 'solo' with it. I fire the two skills on one, then switch mouse, and fire the other accounts, then rinse and repeat until everything is dead except hopefully not me.![]()