I now have two Arduinos talking to one Processing sketch each. I can control parameters of a very simple loop using potentiometers.
Seeing this all together, as simple of a prototype as it is, gives me confidence. It also allowed me to show Oskar and my studio neighbours (Jannes, Mikko, and Matt) my progress to get some ideas. I need to focus on the custom controller and think of inputs other that the midi keyboard and the Monome.
More work on outputs tomorrow.