It sounds more like you want a named event handle per process.
You could use Process.Id
to get a unique ID for each process, and use it to generate a unique string for each process.
Because the process ID is global, it makes it safe to assume that the ID can be used by both processes correctly.
You can then append this number onto some string like "MyProcessEvent" to give, say, "MyProcessEvent6345615". Then use that string to open the named EventWaitHandle
in both processes.
Then your controller process can signal any of the processes it started using that name.
(A process finds its own ID via Process.GetCurrentProcess()
).