The goal of the post was to show that while using both cards doesn't let the RX 580 show video but the computer is able to access it for hardware encoding. So if you need boot screens and an 'encoder' card you could get by with having both cards installed and just not use the RX580 for any display attachments. But the display won't get any accelerated decoding from the RX card as the display would be on the GTX card. You might be able to get accelerated decode from handbrake for example while it is hardware encoding.This mod sure won't able to activate any Nvidia card's hwaccel. That GTX650 has nothing to do with hwaccel, just cosmetic error same as "N/A" if no other GPU installed.
But it's good to know that now allow 3,1 to watch some demanding videos that was not playable, also has much better H264 encoding speed.
When it is a solo RX580 card its great, just lacks boot screen (expected).
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It is curious though that the non-apple iMacPro1,1 is able to airplay without HEVC encode claiming that it works.Airplay not working is a known issue. This function require HEVC hardware encoding, which we still have no idea how to get it work yet.
Maybe that is just a side effect of it being a hackintosh or a VideoProc bug?