Just open the Nvidia control panel. Choose set up multiple displays, and check both boxes, click Apply and answer YES. My Computer.
Why wont it let me uncheck make this my main display?
You have to ensure that you are using the latest version of the graphics driver. You will meet the “make this my main display greyed out” issue if you have incompatible, corrupt, missing, or outdated drivers. To solve the issue, you need to update the driver.