Sketch with multiple classes (.h and .cpp) - how to interconnect classes
In RemoteDisplay.cc you are using Remote in the Initiate method.

Also, since you use the voltage property, you need to include "Remote.h" within (as first line normally) in RemoteDisplay.cpp.

Also it is advised to use within every header file the following pattern (example for RemoteDisplay.h):

#ifndef REMOTE_DISPLAY_H
#define REMOTE_DISPLAY_H
...
...
#endif // REMOTE_DISPLAY_H


This prevents problems that two include header files forever include each other.