This is not a merger of the two app stores. There is speculation that the Windows Store will combine with the Windows Phone Store at some point down the road. That rumor was confirmed months ago by Terry Myerson, the Microsoft executive VP in charge of the software firm's operating systems.
If this report turns out to be true, this would be a small first step toward the eventual merger of Windows and Windows Phone, something that has been rumored since the release of Windows Phone 8.
source: WPDang (translated) via WPCentral