I though this would be a second card for me but because it needs two power inputs I was only to use it as a single GPU in my system. It's been working well and doing exaclty what I bought it for, which was to be able to export 4K video from Blackmagic's Davinci Resolve!
I got one of those to replace my old 5870 in my 2010 12 core 2.93 Ghz Mac Pro and I am really Glad I did, everything is faster now. Finalcut Pro, motion and Compressor are faster and the OS is just more reactive thanks to my new GPU.
I ordered a second one for my 2009 EFI and Xeon upgraded 6 Core 3.33Ghz X5680 Mac Pro and I can't wait for it to arrive. Thanks for a great deal!