On first Nucleus run you have to patch your game so you can run multiple instances of it, follow the instructions in the cmd window that will open, you will only have to do this once, do not click on the Nucleus prompt to open the instances until you have finished patching the game. If you want the instances to resize correctly and avoid stretching you need to create custom resolutions in your AMD/Nvidia/Intel panel (For example: 960x1080, 960x540, 1920x540 etc.) so the game can see and use them. Close Steam completely before running the script. You can also just open Nucleus again and manually select the game exe using the search game button, select it, run it with the number of players you want and you are good to go. Next you need to find where the game's executable is located. Search for this game and download its script. Open Nucleus Co-op, click on Download Game Scripts, the script browser will open.
Download latest Nucleus Co-op and extract it. Tested and working with the latest legit Steam version. We are celebrating 500,000 Nucleus Co-op script downloads and 400 released scripts with another highly requested game!