System Preferences > Displays >> Arrangement >>> Mirror Displays(but know, when MIRROR DISPLAYS is checked, that the display with the lowest resolution determines the maximum resolution used on both, thus one display will have a lower than native resolution, except when both displays have the same native resolution)
System Preferences > Displays >> Arrangement >>> Mirror Displays(but know, when MIRROR DISPLAYS is checked, that the display with the lowest resolution determines the maximum resolution used on both, thus one display will have a lower than native resolution, except when both displays have the same native resolution) Image