On topic:
I think it is no problem to put two R9 Nano's in a Mac Pro.
@h9826790 has experience with two AMD cards in his system, maybe he would want to share how he powered these? And how they behave in terms of crossfire in Windows / dual GPU computing in OSX?
In terms of power, that's relatively easy on the 7950. Regardless what's the TDP of the card, the non OC card only draw around 140W under normal max (around 170W with Furmark). This is base on stock setting ~1.094V, 800/1250MHz. So, lower the voltage to 0.956V (I'd try the minimum stable voltage on both of my 7950, it's actually around 0.9V, so 0.956V give plenty of buffer already. And this is the minimum voltage that allow the card to be further adjust voltage in Afterburner under Windows), this can easily lower the card' power draw to below 125W normal max. In fact, 0.956V already allow you to OC the card a little bit (stablely).
So the card will draw around 40-50W via the PCIe slot, and another 75-85W from the 2x 6pins. Since the 6pins on the logic board can gives out 75W safely (the self shutdown protection will kick in at about 120W). Assuming both card will draw power in similar manner. I now connect them as follow.
Source A -> (mini 6pin to 2x 6pin cable) -> one 6pin to card A and the other to card B
Source B (same as source A).
And then do some quick test in Luxmark will tell if you do it correctly. If you do it right, both source will supply around 75-85W. If you do it wrong, one cable will draw power more than the other (if this happen, shutdown, don't touch the mini 6pin port, just swap one of the 2x 6pin between card A and B, this will balance the power draw).
My old setup was connect one source to one card, that's good for OpenCL (e.g. FCPX), and Corssfire in Windows. However, Crossfire is not always work, and in OSX, OpenGL only work on one card as well. So I now share each card's loading between 2 sources. Which give me the ability to OC the primary card (by software) in Windows when I don't / can't use both card at the same time.
I didn't read the Nano's review yet. If 175W is the Furmark power draw, then it's totally possible to power both Nano by only the 2x 6pin power source (depends on the card draw how much power from the PCIe slot). I personally won't recommend anyone let a 175W card draw power from 2x SATA port. The SATA port provide up to 55W, however, I doubt if the card really draw 75W from the PCIe slot. If it only draw 35W from the slot, another 140W from the cable, then you are drawing 70W from each SATA port. I don't mind to draw a bit more from the 6pins, they are designed to supply power only, and clearly we know the shutdown protection is there, and will kick in at about 120W. But for the SATA port, we have no idea what's the build in buffer, and if there is any proper protection.
Assuming I will only kill the port but not the whole logic board by drawing too much power. I prefer to kill the mini 6pin. There is an easy work around to draw power from the PSU again by doing the Pixlas mod. But once I kill the SATA port, that's done, no work around.
