I have a Windows Phone 8.0 (Silverlight. Not RT) App in the store. Of course it works with wp8.1 as well.
No I also have a new Windows 8.1 App that does the same but for tablets, notebooks and/or desktops. I'd like to have them in the store together, so people see "Ah, I can download it for the other device as well!" as both are free.
How can I do this without recreating my Silverlight WP app in wp8.1 RT and then combining them into a big universal app?
The most important point for me is not having 1 solution in Visual Studio but having the app connected in the Windows Stores.
This works the same for Silverlight as it does for Windows Phone Runtime apps. You don't need to change the app itself. You can update this fully on the dashboard:
Log in and select your existing app. In the LifeCycle pane create a new submission or edit your existing one Edit the App Info to set a name which matches the associated Windows Store app.
See How to associate an existing Windows Phone app to an existing Windows Store app
If you're just adding the new Windows Phone app you can do essentially the same on the Windows Store dashboard. When you upload your Windows Store app and reserve its name you can select one which matches your existing Windows Phone app.
See Sharing your app’s identity in the Windows Store and Windows Phone Store