Android Question Coding for devices with more than one screen.

And the Surface Duo - which I anticipate purchasing as soon as it is launched

I believe that this is the relevant API: https://developer.android.com/reference/android/app/Presentation

This will require building a library.

I believe that Presentations are a specific kind dialog

A presentation is a special kind of dialog whose purpose is to present content on a secondary display

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A more general use of two screens by Activities is here
I think that to support the general case of multi-screen Activities would need changes to B4A core libraries.