how to set up 2 displays

Take a look at > Use external monitors with your Mac - Apple Support

Hi,

We need to know more about your iMac model which is available Apple menu > About this Mac.

I connected a 27 inch Acer display to my Late-2013 iMac 27 inch via DisplayPort cable. With the external display powered, you will get an additional tab in the System Preferences : Displays that is named Arrangement. I arranged the iMac display on the left, and the external display on the right (where it is physically). With this setting, I can just drag the mouse to the right onto the external display.

I happen to use Virtualbox and Windows 10 (1903). Virtualbox will use the external display for its guest operating systems.