Keep Mirroring as default

I have a Mac Mini to which I connected an external display (24") and a rudimentary keyboard.
I also have a McBook Air which I like to use as Keyboard, track pad and as secondary screen.
I set my Mac Mini to never sleep, so when I turn my external screen, it is on with the same apps opened. Luna is always on on the Mini.
Then when I come home, I open my MB Air and turn on the Luna App and both screen are now available. However, they are separate screens (not mirrored) and I cannot access the external screen of my Mini from the MB Air as the cursor of the MBAir cannot access the screen of the Mini (and if I don’t turn the external Screen of the Mini on, I cannot use my MBAir screen as the screen for the Mini.
The only solution I found so far is to use my rudimentary keyboard attached to my Mini, access the “display” link in the System Preference, and then click on “Arrangement” so I can click on “mirror display”. Then I can use my MBAir as both keyboard and track pad to run my Mini - and set aside my other keyboard.
What I want to solve is for my Mini to be set as “mirror display” as default, so when I open my MBAir, turn Luna on, and now I can immediately operate the Mini from my MBAir.
How can I do that?
Thanks for your help :slight_smile: