How to send button down to a button of another WPF Application

Question

I wrote an WPF C# application (called slave application)  that have a button. When click on it, it will perform a task.
Now I launch 10 copies of this slave application.
I will like to be able to create another C# application (called master application) with a buttton. When click on this button in the master application , it will as if I have manually clicked on each of the button of the 10 copies of the slave application.

Did anyone have any idea how I can achieve that? Any sample program that demostrates that?