Two Apps running with cross communication between them

I need a way to run two apps and have them communicate with each other. First App will have control buttons and when pressed must be able to send this to the second App so it will respond to the button clicks from the first App. The second App must be able to send results back to the first App.

The Apps are running in real-time.

I don't need the code (although small examples would be nice); just some ideas.