There are some technical and accuracy issues that I am sure the Enginners will explain.
Perhaps what Astropad, could one day, do is offer a floating resizable window that could contain reference material. Til then you can cascade two views in Photoshop or show reference material in the graphics library pallet also in Photoshop.
An alternative would be to use two physical monitors on your Mac. I bought a 20 inch fell ultra sharp for just 100 dollars. It was refurbished but makes a great second display that matches the iPad pro’s 3:4 aspect ratio.
Third option, use two iPads. One as a second USB display via Duet or Air Display. Then use Astropad via WiFI to mirror your main display. Disclaimer: For my last bought with Duet, it created a blank Menu Bar and was not compatible with the latest version of Photoshop CC which produced a blank Canvas. That’s one of the big +'s with Astropad. It just works.
Fourth, AirPlay with AppleTV to show reference Material.
Fifth, for second monitor get an actual USB display. This is one option, I have not delved into.