I'm working on a port of my WP 8.1 app to run on tablets, and PCs. I started working on a Windows 10 port, but I realized that it won't run on Windows Phone 8.1 devices. As far as I know the other way was to create a Universal 8.1 app, but that would mean to redesign most of my pages, there would be a bunch of API and control incompatibility with WP and Windows 8.1. (Pivot control does not exist in Windows 8.1). That's why I chose the easier way to port to UWP. The question is: can I upload a Windows Phone 8.1 package, and a Windows 10 package (only PC and tablet device families targeted) to the same app in dev center? So both mobile and PCs, tablets are supported.
