Windows 8 and Windows Phone 8 have a common software foundation, but their app libraries are currently separate. That could all change soon. Quoting sources wise to Microsoft's plans, The Verge reports that the company intends to consolidate its app stores for the two operating systems.
According to The Verge, this plan was announced internally by Terry Myerson, the head of Microsoft's newly formed operating systems group, during a meeting "attended by thousands of Microsoft employees." Myerson said the app store consolidation would take place upon the launch of the "next releases" of Windows and Windows Phone. According to The Verge, the executive was referring to Windows Phone 8.1 and a "special update planned for Windows 8.1," both of which are scheduled for next spring.
Not many other details leaked out, so it's not clear exactly how the app store consolidation will go down. As The Verge points out, Microsoft could copy Apple's approach, allowing Windows users to download and run Windows Phone apps, but not vice versa. Myerson has dropped some hints about phablets recently, though, so it's possible that something a little different is in the cards. I suppose we'll find out soon enough.