How to mirror a first screen on the second and keep the highest resolution on the first screen?
In order to access Mac OS interface from another screen, go to System Preferences > Displays > Arrangement and take a look at the blue rectangles that represent the screens you currently have connected. One of them should have a white bar at the top, drag this bar to the screen that you want to access the OS interface from. That way you can keep the higher resolution, and access the interface. Unfortunately, if you mirror a display, it will take the lowest resolution because it cannot "create" the pixels required on the lower resolution screen in order to show a Mirrored display. If you want to keep the higher resolution, you have to extend the displays.