Surprisingly or not, the ways of doing communications between application of the same computer are nearly the same as between application on different computers. Different processes are isolated and need inter-process communication. First, I would suggest either classical remoting or self-hosted WCF using IPC channel, which is based in Windows named pipes.
See:
http://msdn.microsoft.com/en-us/library/kwdt6w2k%28v=VS.100%29.aspx[
^],
http://msdn.microsoft.com/en-us/library/dd456779.aspx[
^].
You can also use networking on different levels which I overview in my past answers:
how i can send byte[] to other pc[
^],
Communication b/w two Windows applications on LAN.[
^].
Using the same method for communication between processes on the same of different computer is a good thing. Today they are on the same machine, but how do you know what will you do tomorrow? The best for you would be just changing configuration without changing your main code.
—SA