The relationship between two programs in python

I have 2 programs that are in python, one of them is a third party and has already been done. I want to run one in the foreground and one in the background. I want to configure the connection between the two programs, so I can say that in the foreground to do something, and then tell the other program to do something. For example, in the foreground program, I press a button that then tells the other program to do something. What is the best way to do this?

I know I asked a question about this before, but I feel that people may have found this too specific, so I wrote a more simplified question.