Print one screen in dual monitor - Ubuntu 20.04

Please go to keyboard shortcuts

Settings -> Devices -> Keyboard -> Keyboard Shortcuts -> Screenshots

For me as per the above keyboard shortcuts, Ctrl+Alt+Print copy the currently active monitor screen window to the clipboard. If you want to take another screen monitor window, just do mouse click in the other monitor window to activate and then press Ctrl+Alt+Print. Ctrl+Alt+Print only copies the screenshots to the clipboard. If you want to save the screenshots, just press Alt+Print.

I hope it helps. Thanks.