I received a response from EVGA that seems at odds with info in Otoy’s FAQs:
EVGA:
Is the mother board on that Mac SLI compatible? For 2 cards in SLI the second card only helps in processing and does not use any of the VRAM so 2 2GB cards will only use 2GB. I would recommend just using 1 680 in the Mac Pro as I am not sure if Macs are capable of SLI. You would need to check the motherboard model to see if it has SLI compatibility.
However Otoy says that SLI is not needed and that using multiple cards is the way to scale performance:
Yes! Octane Render completely relies on the GPU for rendering performance and scales extremely well. If your motherboard can accept more than one video card, adding additional video cards will greatly improve Octane's rendering speed because Octane's performance scales perfectly with the number of GPUs (e.g. rendering with four GTX Titans will be 4x faster than using only 1 GTX Titan), without the need for SLI.
Does Octane Render take advantage of SLI?
No, but it can use multiple video cards for rendering (see above). It is also recommended to disable the SLI option in your Nvidia control panel to maximize Octane's rendering performance
Can anyone clarify if two GTX 680's for Mac) in a MacPro 5,1 with a single 27" display is a workable configuration?
(One card connected to nothing)?
Wouldn't two GTX 680s be a better choice for Octane than a GTX 5000 for Mac?