Hi

I am not entirely clear about your question.

If you are asking how to build and deploy each project then it is important to right click the project name in the solution explorer and click "Set as Startup project". Then F5/Start will build and deploy for arduino or for windows.

You can also right click project names and you will find individual build and deploy options so you can experiment with the best workflow for you.

There is not need to have both projects in a single solution. You might find it easier to open the ide twice and load one project into each.

If you are asking how to make a windows program communicate with an arduino using serial then you need to use sites such as stackoverflow to answer your questions. In short the c# windows app will have a SerialPort either via the toolbox it one exists of defined in your code after importing the System.IO namespace and using SerialPort myport = new SerialPort(). Don't forget to read about setting the correct Baud rate (speed) to match your arduino Serial.begin() code.

I hope this helps.