This is slightly related to the previous FAQ on connecting to localhost. While Metro apps can use contracts to communicate at some level with other apps, that is not equivalent to the traditional concept of inter-process communication. A metro app cannot communicate with a desktop app or even with another metro application on the same machine.
With desktop apps, the metro app cannot take for granted that it’s running or available, and thus it makes sense to not allow that. But you may be wondering why IPC is blocked between two metro apps. Well, metro apps can be in a suspended state at any given time, and there’s no sure way to predict its state. So even if there are two apps running at the same time, neither app can be sure if the other app’s ready to accept data or a command. This is quite likely why they decided to design it so that metro apps cannot talk to each other.
So, what’s the workaround? Again, the only reliable way an app can talk to another app on the same machine is to use a a 3rd service that’s running on the network, or to use a cliched phrase, use the cloud.