So I found something interesting-and fun, but I'm limited in testing due to only a solo Cinema Display, but want to test both engaged simultaneously to see if the RX580 becomes displayable in macOS... Full Metal Acceleration on both a GTX770 and RX580 in both Mojave 10.14.3 and Win10; I am running Bootrom 126.96.36.199.0 in my MP 4,1->5,1 upgrade. My NVIDIA GPU is a MVC flashed GTX 770 Founders Edition 2GB with bootscreen-and reports correctly as a 770 since I purchased in 2016. Doubtful a MVC original-but the bios is locked; I got it from an east coast seller. Possible it was just re-sold to me though. I am presuming any of the GK104's with the EFI ROM should function the same-but am guessing at this point. My AMD GPU is a Sapphire Pulse RX580 8GB standard dual bios OOTB, using the primary bios. I was very happy with this setup's overall perfomance in both macOS and Window but sorely missed Bootscreen and PhysX in Windows. Both cards are currently limited via Afterburner in Windows 10, and self-regulated in macOS. Use caution-you can still trigger shutdowns if only using the PCIe boosters. Have a plan to provide power before even considering dual GPU's of any configuration long-term. The RX580 in slot 1 was my first attempt last night, which worked-but with the same limitations I have now, but the RX580 would run hotter due to Chill engaging (but not getting hot enough to trigger the fans). The 580 in slot 1 idles at 42 deg C, but in slot 2 it will idle at 38 deg C (prob cause the GTX FE's fan is circulating right next to the RX580's heatsink). Also, the GTX required a stand-off when in slot 2 to prevent the RX's fan's from clipping the solder-point's on my GTX which does not have a backplate. A secondary case and PSU could be warranted-but even without display in macOS, the RX580 is providing accelerated compute that is visible by both app and monitor which are the main reason most of us use them and is why I absolutely consider this a success. The dreamer in me thinks a properly powered Vega and GTX combo could possibly pump some serious frame numbers; but I still haven't been able to push this setup fully yet. I have a theory the AMD is functioning as it's supposed to in the presence of a EFI NVIDIA GPU, solely as a booster compute GPU in Mojave. NVIDIA's have been used for years as PhysX compute cards to an AMD GPU in Windows, and why I began exploring dual GPU use. Apple, it appear's, decided to implement something similar but vice versa in their compute cards; from what it looks like to me. It may be tied to eGPU applications, but I have no way to set which GPU to use for individual programs like eGPU's do under Show Inspector menu "Prefer External GPU" checkbox. I only sent a 15 second clip from FCPX to Compressor, and had "Use GPU to process" in Compressor's Advanced Preference's and it shared the load natively. I've not run any heavy testing in macOS yet beyond basic editing and compression with FCPX and Compressor due to not having a secondary PSU or Pixlas mod available (yet). In macOS: 1) GTX770 must be used as primary-DP is functional (but when swapped to RX580 is not). 2) CLing is reporting Metal acceleration in both GPU's 3) FCPX does not appear to use the RX580, but Compressor does as a booster GPU. 4) Heaven uses only the GTX770 (or I have just not been successful in engaging the RX580 yet). 5) I am hoping a second monitor may change #3/#4 (or future software updates?). In Win10 (cards are currently power limited via Afterburner to 120w (770) and 129w (580)): 6) Best to use the RX580 as primary-both cards are usable. 7) GTX770 functions perfectly as a dedicated PhysX/CUDA card when using RX580's DP. 8) Certain settings on each AMD/NVIDIA control panel are only accessible when a monitor is connected to each manufacturer's card, so PhysX must be set to the GTX in the NVIDIA Control Panel prior to swapping to RX580 for use in Windows. It only needs to be done once usually, unless you move the cards slot location. 9) Ensure your cards are properly powered or you are comfortable limiting your cards in power somehow before attempting. I have a mDP switch that I've re-purposed (previously was buggy at times when using w/ my MBP, so I don't recommend it by name brand), that's working well for now between GPU's. When setting up I was just swapping the DP cable by hand once booted to Windows between the GTX/RX where hot-swaps aren't a problem. They aren't a problem in macOS, but again-you just don't have a display via the RX580 in macOS. I am planning a Pixlas mod to eventually power the RX580, or secondary PSU for both (undecided)-but wanted to share this setups viability for those that already have those bases covered. I personally favor a secondary PSU for longer-term use, but this is certainly viable for my use in Testing.