You can add the name of the library (as used in the import statement) in front of the class. For example promidi.Controller or controlp5.Controller so the comp knows which one you are talking about.

Ah -- brilliant, and easy. Thanks -- and thanks double for the quick reply!

To differentiate between the two classes you need to apend the package name.

In most cases this is very simple when you include the library into your sketch it adds one or more import statements. For instance ControlP5 adds the line

import controlP5.*;

so instead of
Controller c;
use
controlP5.Controller c;

Same idea for the other library which will have a different packge name