"Disable the second monitor in Display Settings, take screenshot, re-enable second monitor."

If you're using Windows, you can also use the Snipping Tool to screen print only one monitor:

If you are a designer or developer, the built-in tools might still feel clunky. Enter (Windows) or CleanShot X (Mac).

This is the secret weapon for Mac users. Press Command + Shift 4 , but don't drag. Instead, press the Spacebar .