Intercept messages between processes

Hello, I got a program A, and another program B. In B its possible to create stuff that can then be run with A. I have then decided that I want to create my own program C instead of B. My problem is, that when B wants A to run something, and A is already running, I have no idea how B communicates with A.

In case you want the specifics, A is Starcraft II, B is the Starcraft II Editor, and C is an editor for an extension of the galaxy scripting language. It's all happening on a windows machine.

TL;DR: Does anyone know any good tools/methods to watch/analyze the communication between two processes?