Solved! searching through the settings, I discovered that it is possible to set directly in App Designer the unique instance for each single App: this therefore allows you not to open the same App multiple times, but to have only one App (Secondary App, in my case) open.
Exactly what I was looking for, simply by enabling a check box in settings.
Here is what to do for those who find themselves in the same problem as me:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1724141/image.png)
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1724146/image.png)