Crossfire resolution and use of multiple displays

I'm wondering, since I use two monitors, one is at 2K resolution while the other is at 1080p FHD, if that causes me problems when using the default shader. Now I have a theory, whether it is possible to reduce the resolution of the game to FHD from 2K, because I would still use the main monitor to display the game. I'm asking because I don't see the option anywhere in game options resultion is locked to 2K, I even checked "low resolution" in launcher options and nothing happened, I still can't lower the resolution. Is there somewhere in the folder where the game is located some configuration file where the resolution can be changed without ruining the game itself. Also, I wouldn't risk being banned from the multiplayer, that's why I haven't tried to tamper with the files so far. One more thing, is the resolution controlled by the main game or Crossfire? Thanks in advance for any response

Edit: I tried to launch the launcher itself on the secondary monitor, but the game still launches on the main one.

Another thing when installing Freelancer I had the option (since I'm using a repack version of freelancer adapted to higher resolutions) to choose a higher or default resolution. I chose high resolution, should I try to reinstall freelancer and select the vanilla version and then install crossfire again, would that affect the resolution selection?

Ok i remember that from times before crossfire when i tried to change reso by hand I did manage to lower resolution but have to uncheck borderless window mode as a matter of fact i have to uncheck all boxes otherwise i get game on three quarters part of display maybe that is good for something who knows. I'm left to hope that some update doesn't return these settings to the old ones, and I miss borderless window-mode a little, but you can't have everything, right?Yet i'd like to thank you for your answer it's been helpful.

Yep i was right changing resolution does the trick stable 144 fps and exelent frame time in all situation even in MP. My graphic card can deal with shaders but that diferance in reso between 2 monitors somehow affects the fps and causes micro stuttering and lags, even though my GPU consumption is minimal. I have just one more question. maybe I'm asking too much, is it possible to add an option to choose the resolution in the launcher or the game itself, that would finally solve all my problems