This is reposted from another thread, hopefully more eyes will see it and maybe make sense of it. In my 2009 MP, flashed to 5,1, I have tested dual K5000 mac edition cards, and 680 mac edition in the case, and two K5000 in an nvidia quadro plex. They work almost correctly in el capitan, but not in sierra, or high sierra. When using the built in high sierra driver, with for example two K5000 inside the tower, if I connect one cinema to either card, things are fine. If I connect two cinema to either card, things are fine. If however, I connect one display to each video card, when I plug in the second display, (dvi or dp), high sierra suddenly logs me out, and goes back to the login screen. If I try to login, it will load the desktop for a moment, then log out again. If I unplug one of the displays, things are fine and I can login. If I load the newest nvidia driver, including the one that just came out, things are different: Only one card will have desktop acceleration, and its never the card in slot 1, its the 'last' card in the chain. Example: Two K5000 inside the tower - only card in slot 2 has acceleration, this includes 2 displays on it. Any display plugged into the slot 1 card has no acceleration and stutters/bad color/graphic errors. This happens even if I switch the cards around, only slot 2 works correctly. One 680 mac edition in slot 1, pcie breakout card in slot 2, two K5000 in quadro plex expansion unit: Testing with multiple display shows that only one card has acceleration, and its the second card in the expansion unit. The 680 has none, and first slot in expansion has none. In Sierra problems were a little different, but the computer was still unusable, as it would completely crash and reset about every 15 minutes when multiple displays were connected to separate cards. In El Capitan, things seemed ok, and I thought I had the problem beat, but no. Under heavy stress testing, meaning one display on each of 3 cards, and Heaven 3d benchmark running on each card with its own display, it ran for about 15 minutes, then the computer black screen crashed and reset. I havent tested this yet, but I think if I put the pcie breakout card in slot 1, a 680 mac edition in slot 2, and two K5000 mac edition in the quadro plex external, then I can connect displays to the 680 with acceleration, and the two K5000 will be available for cuda, but no acceleration for any displays on them. This has been EXTREMELY irritating, as I have spent literally hours trying different setups drivers, and three different versions of mac os.