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I got this working on Windows 10 on bootcamp on Mac Pro (Late 2013) purchased in 2014 with this post (also scroll down there for the audio fix): https://community.amd.com/message/2661642#2661642

Here's a video I made with my phone of a benchmark with this set-up running "Shadow of Mordor":


It has the basic optimizations mentioned in this thread, such as disabling ULPS, and I verified the simultaneous operation of both cards with MSI Afterburner.
 
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