With Windows 8.1 Update, we’re pleased to introduce two features for side-loaded Windows Store apps that make it possible to interoperate with desktop processes (apps or local services) outside of the app container. First, is the ability to use network loopback, and second, a brokering mechanism that allows you to mix desktop and Windows Store app code within the same app.
Both features are fully described on the Enterprise page of the developer center, from which you can also download two complete white papers and sample code.