How can you add a second display to a MacBook Air to extend the view and having the mouse navigating from one screen to another while leaving the laptop closed?

Hi everyone

I am working a lot on editing and have two display screens. I have a docking station that powers the laptop and connects the two screens via HDMI to the docking station. However, the display setting shows me only one screen as an extended display and the screen from the laptop. I'm using the MacBook Air as a mirror to display 1, but screen 1 do precisely the same as screen 2, but I want screen 2 to be an extended view of screen 1. How do I do that? Thank you very much for your help.

have a fantastic day ;-)

Fred