Serial port connection with two C# application

Start by making a physical connection: you will need an RS232 line between them both.
Then, use Hyperterminal (or similar) on both machines to ensure that you have identified the communications parameters correctly, and that the link is working fine.

Then all you have to do is set up a SerialPort Class[^] on each and handle the DataReceived event.
You can then start sending from one and receiving on the other, and vice versa. (The link contains basic sample code)