reading and writing to files from GDL is only good for communication with external programs.
The problem with communication between library parts is that the order in which the scripts run is not deterministic. There is no guarantee that the current parameters will be written to the file before the other libpart will try to read it.
The only way for communication between library parts is to write an Add-on that sets and reads parameters of the library parts based on their subtype.
This is beyond GDL, C++ programming experience is required. You can join the API developer community at https://archicadapi.graphisoft.com/start.
Library Platform, Software Engineer