You can have multiple documents open in AR and cycle between them, but you can only see one document at a time.
So although you can’t drag content between them you can Copy & Paste content between them easily.
If you need to actually see two documents simultaneously on screen, so as to work on both side by side, then running more than one instance of AR would be needed. But again you would still have to use Copy & Paste to transfer content between them.
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Techie Stuff:
ArtRage 6.1.1 ~ 15" Macbook Pro ~ macOS 10.14.6 ~ 4 Core i7 3.1GHz CPU ~ 16GB RAM ~ Wacom Intuos4 M