When two processes need to communicate, they do so using one of any number of inter-process communication (IPC) mechanisms. The answers to this SO post give you a few ideas of the possibilities. To this I would add WCF, which is the Microsoft-recommended way of having two .NET-based applications talk to one another.
Personally, I choose sockets for this kind of thing, but it really does come down to your particular situation and your comfort level.
HTH